So the question is
does schizotypal personality disorder explain lifelong homicidal ideation / obsession, or predisposition to same?
Schizotypal personality disorder does include paranoid ideation among its symptoms. Paranoid ideation has been linked to homicidal behavior in some studies
Patients who reported homicide attempts could be distinguished from patients with no homicidal behaviors by the presence of other aggressive behavior such as suicidal ideation and suicide attempts by themselves and their family members and by elevated current measures of interpersonal sensitivity, hostility, and paranoid ideation.
As to whether it's enough to "explain" such behavior... that's a tricky question as it depends on what you mean by "explain". If you interpret "explain" as "makes it more likely", a psychiatrists would say yes. If you interpret "explain" as "excuses in the eyes of the law", a jurist would say no.
According to Wikipedia (which does not cite any ref for this)
Most courts accept a major mental illness such as psychosis but will not accept the diagnosis of a personality disorder for the purposes of an insanity defense
Note however that the distinction between personality disorders and the rest (formerly axis I) has been abolished in DSM 5. So this is an area of fairly recent changes in psychiatric thinking. I don't know how courts will react (or maybe some have done so already) in reaction to this change in the DSM. N.B. you'll probably want to read https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3811091/ for more on the criminal law and personality disorders; it's written in 2013 (so after the adoption of DSM-5), but doesn't mention any cases where the new categorizatoin has influenced criminal-law decisions. I guess it was still too early for that. See also http://jaapl.org/content/42/2/141.long which is more focused on the DSM-5 change. From the latter is worth quoting:
The DSM-5 contains an expanded Cautionary Statement of explicit relevance to forensic psychiatry. It states in part that, “In most situations, the clinical diagnosis of a DSM-5 mental disorder … does not imply that an individual meets legal criteria for the presence of a mental disorder or a specified legal standard… .” It goes on to state that, “a diagnosis does not carry any necessary implications regarding the … individual's degree of control over behaviors that may be associated with the disorder” (Ref. 3, p 25).
So again a psychiatrist's view and a the law's view are different. Note than in mostly non-US (but still English-law-based) jurisdictions, there's a notion of diminished responsibility, often applied in cases of mental illness contributing to a crime. The US law use of this notion of diminished responsibility appears very limited; it seems to be used mostly in California (but not in Colorado where Holmes was tried.)
And since we're talking serial killers with psychotic tendencies it's probably worth recalling the case of Breivik and Norwegian law. It's hard to summarize that case here since it's an entire saga with front-page disagreements on the diagnosis etc., as well as peculiarities of the Norwegian law with respect to mental illness definitions. But in the case of Breivik, just like in that of Holmes, there was no agreement among psychiatrist what his diagnosis should be. In Breivik's case the court ultimately accepted the personality disorder diagnosis rather than an axis I diagnosis proposed by some psychiatrists. In the case of Holmes the alternative diagnosis was schizoaffective disorder, offered by another court-appointed psychiatrist. Given the difference in law between Colorado and Norway, it's less relevant what Holmes' actual diagnosis was; in Norway a psychotic disorder automatically qualified as incompetence to stand trial, but not in Colorado, where the M'Naghten test (telling right from wrong) applies.
And coming back to the meaning of "explain", the latter psychiatrist who examined Holmes said:
Metzner, who diagnosed Holmes with schizoaffective disorder, said he did not think Holmes went on his rampage to get notoriety or because of his longstanding hatred of mankind, which he described in a spiral notebook. Instead, Metzner said, Holmes' actions were "directly related" to delusions that killing people would increase his self-worth.
"I can come up with no other explanation for what he did," Metzner said.
Holmes knew what he was doing was illegal and that others would think it was wrong, Metzner reiterated in response to questions from District Attorney George Brauchler. The prosecutor brought up Holmes' statement that he did not want to date someone because he did not want anyone to be featured in news coverage as the girlfriend of a mass killer.
And paralleling the Berivik case, the judge didn't dig this
[Judge] Samour ordered a second evaluation after he found Metzner’s report was “incomplete and inadequate.”
And regarding some of the subsequent questions
Is it likely that such ideation could be kept secret for one's whole life?
Schizotypal personality disorder also has among symptoms a "reduced capacity for close relationships" usually manifested as "lack of close friends". So that makes it more likely such individuals will not have many people they could talk to about their weird ideas, so they may not get noticed until it's too late.
From the unsealed notes of Dr. Fenton and Ms. Roath
Holmes had been referred to Dr Fenton by University of Colorado social worker Margaret Roath in March 2012, who noted it was “hard to interview him as he would just start and take a long time to answer”.
“When I asked him about other symptoms, he said he did not want to say as I would have to report him. He says he wants to kill other people, but no one in particular and has never done anything to harm others,” Roath noted.
A note of Obsessive-Compulsive Disorder was accompanied by the description that “(Holmes) has problems concentrating, eats OK, does not have crying spells and does not have thoughts of suicide.”
In an email to Dr Fenton to see him “soon if possible”, Ms Roath described Holmes was the “most anxious guy I have seen”.
“Most concerning is that he has thoughts of killing others people, though I do not think he is dangerous,” she said, adding Dr Fenton would see him as it “might be best to keep him in the system”. [...]
In her notes, Dr Fenton expressed concern over Holmes’s homicidal thoughts but his lack of a willingness to address specifics with her. He told her if she revealed more details she would “lock him up.”
Those in charge of his courses has told her of his “extremely awkward” social interactions and “odd comments”, but “neither had heard of him making threatening remarks”, she noted in a June client summary.
A background check had revealed no weapons permits, she said.
“At this point. James is … intermittently functioning at a psychotic level,” Dr Fenton wrote.
But she concluded he “does not currently meet criteria for a mental health hold”.
“He is not gravely disabled and has no evidence of suicidal ideation. Longstanding homicidal ideation but denies any specific targets and there is no current evidence he is angry at the grad school (or anyone else).”
She noted that while Holmes had made hostile remarks to her and another psychiatrist, he had not made threats.
Captioned: "Psychiatric notes released to public. This one, a mental status exam, by Dr Fenton"
So your (2nd) question becomes mostly hypothetical in view of this. Also, whatever obscure statement he may have made to a friend about "floodgates" pale in comparison to the more direct statements made to mental health professionals before the murders, although had a professional heard those, they might have seen it as a sign he was going to act. We can only speculate at this point what some professional might have done corroborating all the evidence.
As an aside, Fenton was sued in 2013 by some victims' relatives. Fenton did warn someone on the campus police:
The documents show Fenton went to University of Colorado police officer Lynn Whitten and informed her that Holmes confessed homicidal thoughts to her and that he was a danger to the public, "per her requirement," the documents say. Whitten responded "by deactivating Holmes' key-card access to secure areas of University of Colorado medical campus buildings," the Denver Post writes. Fenton also told Whitten that Holmes had "stopped seeing her and had begun threatening her via text message," the documents state.
There's a bit more of the campus-level story:
Holmes started seeing Fenton in 2012 to help with his social anxiety. He soon confided that he was having thoughts about killing people.
His last visit was June 11, 2012. By then he had already amassed an arsenal of weapons and body armor and was meticulously planning his attack on a packed showing of a new Batman movie. But all Holmes told Fenton and another psychiatrist, Robert Feinstein, was that he had failed a key exam and was dropping out of the neuroscience program.
Holmes "left in a huff," Fenton told Whitten, whom she called the same day. She was concerned because Homes had told her "I want to kill as many people as possible," and she wondered if he was nearing a psychotic break, according to the documents.
Though he had "constant homicidal thoughts," Fenton told Whitten, he had never revealed specific targets or plans. But Whitten wrote in her notes that "Holmes indicated to Fenton that he liked thinking about it."
Fenton was alarmed enough to violate Holmes' health privacy rights and call his mother, Arlene. But Fenton did not tell Arlene Holmes about her son's homicidal obsession, instead mentioning that he'd dropped out of school.
The documents show that Fenton found Holmes' mother's reaction reassuring.
She left a voicemail for Whitten: "It's pretty good news on our guy James. It sounds like he's always been like this. I've talked to his mom, she and the dad are kind of putting some quiet support in place. They've got my number in case they need anything."
She concluded: "So, anyway, I think we've got everything in place for now, so if something else comes up I'll let you know."
Arlene Holmes testified during the trial that Fenton didn't return her calls seeking more information. Fenton chose not to detain Holmes because she felt he was "on the borderline" and didn't think it would help, she told Whitten, according to the documents provided by the university.
About the same time, Fenton called University of Colorado professor Sukumar Vijayaraghavan, head of the neuroscience program, about Holmes, according to documents provided by prosecutors.
"James had told her (Fenton) he did not think he would make a mark on the world with science so he could blow up people and become famous," Vijayaraghavan told Aurora police after the shooting. Vijayaraghavan said he then called Whitten, who told him "action would be taken immediately." She said she had suspended his student ID and "police are aware of the situation and are keeping an eye on the whole thing."
After the shooting, Whitten told Aurora police she "discussed Holmes" with her sergeant and checked with others on the status of his withdrawal from the university.
The university said in a statement that its faculty and staff acted responsibly. Campus police officer Lynn Whitten, who was not named in the lawsuit, has since retired. Her attorney, Tom Rice, said she did all that she could with the limited information she had, none of which gave her reason to believe Holmes was an imminent danger.
From this one can guess that the civl lawsuit against Fenton didn't get far because:
Under Colorado law, mental health professionals cannot be held liable in civil suits for failing to predict a patient’s violent actions unless it involves a “serious threat of imminent physical violence against a specific person or persons.”
And I was right on the money, the lawsuit against Fenton was dismissed in 2016.