Since I vaguely remember a similar question not that long ago, let me use this one to quote from one the few peer-reviewed papers (doi link; preprint pdf). that discusses the notion of "mind control":
Mind control is a common plot device in many genres of fiction.
Its ubiquity is perhaps unsurprising: the prospect of
the explicit, full control of the mind evokes alluring and
startling possibilities. Fictional mind control often takes implausible
forms: telepathy, magical interventions, and nefarious schemes of
authoritarian organizations. In more biologically inspired plot lines,
mind control is delivered by devices implanted in the subject’s brain,
as depicted in The Matrix: these devices manipulate neurophysiological
processes resulting in a change of mental state.
In reality, mind control encompasses numerous means for
influencing the mind. These include effects mediated through the
senses. Sense-mediated effects can include positive influences, such
as updating one’s beliefs based on presented evidence, or nudging
someone to make healthy decisions via environmental manipulation.
They can also be more insidious, as in the case of propaganda
or brainwashing. Beyond social and environmental means, mind
control can also result from direct neural stimulation. Neural stimulation
can include subtle modulation via pharmacological agents
or more direct manipulations with brain stimulation that result in
neural discharges. The last few decades have seen a steady increase
in the use of implanted devices to assist individuals with major
mental disorders. The ubiquitous nature of social forms of control
and increasing prevalence of neural devices motivates important
questions about the control of brain processes and, by extension,
mental functions.
So as you can see other things qualify as (partial) mind control just as well, deep brain stimulation, rRTMs etc., as well less invasive ones, propaganda etc.
[-much later, the paper talks about ethics-]
Insofar as mind control has been and will be undertaken in
experimental and clinical contexts, four basic principles of medical
and research ethics apply: non-maleficence, beneficence, justice,
and autonomy. Adhering to these principles is a first step toward
ensuring that efforts to guide the mind enhance human welfare
without violating human rights.
So it's less of matter of means than of context and motives what is unacceptable mind control. Think for a sec how is indoctrination different from psychotherapy, for instance. And regarding "HOW can anyone trust a hypnotherapist if they have the ability to slowly word their way around your belief system one suggestion at a time?" The latter doesn't require hypnosis. You can ask exactly the same question about psychotherapy. That's why stuff like sex with patients is prohibited by codes of ethics.
And since I've not talked at all about hypnosis above let me say that traditionally the literature about hypnosis (at least the one with therapeutic bend), emphasizes that people cannot be made to do anything they don't want to under hypnosis. Typical of this is the Sci Am Mind magazine article "The Truth and the Hype of Hypnosis", which has been quoted in some textbooks for the received wisdom that:
"(if you think) People who are hypnotized lose control of themselves. (the reality is) Subjects are perfectly capable of saying no or terminating hypnosis.
On the other hand, recent (2017) research by Lush et al. does indicate that thre's perhaps an objective measure of a "feeling of involuntariness" in hypnosis subjects.
Using an established test designed to measure self-agency – the sense that our own actions are actually down to us – participants were shown to be more likely to feel their movements were involuntary after hypnosis.
The actual metric of self-agency relied on measuring intentional binding. Quoting New Scientist's description of the experiment (as the most concise I found):
Lush’s team asked 18 people who were highly susceptible to hypnotism to sit in front of a very accurate clock and do a task repeatedly where they pressed a button, triggering a beep after 250 milliseconds, in three different circumstances.
If their finger was pulled down involuntarily – by an attached string – they perceived the period before the beep as 176 milliseconds. If the string wasn’t pulled and they chose when to press the button, they experienced it as 91 milliseconds – showing intentional binding.
But if they pressed the button due to a post-hypnotic command, they perceived it as 156 milliseconds – closer to the involuntary state than the voluntary one.
“This feeling of involuntariness is a key element of the hypnotic experience,” says David Spiegel of Stanford University.
A somewhat similar (2017) study by another research team (published a few months later), but which has the advantage of using a control group (but the disadvantage of not doing actual hypnosis):
Here we tested the proposal that distorted volition during response to suggestions arises from poor metacognition pertaining to the sources of one's control. Highly suggestible and control participants completed a motor task in which performance was reduced through surreptitious manipulations of cursor lag and stimuli speed. Highly suggestible participants did not differ from controls in performance or metacognition of performance, but their sense of agency was less sensitive to cursor lag manipulations, suggesting reduced awareness that their control was being manipulated.
Since these studies are too new to have been covered by a review, if I may inject my own interpretation here, it does look like hypnosis works as a "double whammy" in highly suggestible individuals: not only to do they start with a lower metacognition of self-agency, but this is further lowered by hypnosis itself.
Moreover, recent "revival" research in hypnosis has confirmed that LSD enhances
hypnotic susceptibility,
and a new finding is that rTMS can also do the same. You should be very worried now :0
Seriously, I mean to show by this that there's a continuum of "mind control" and while hypnosis surely is an interesting phenomenon in that regard, it's not the "OMG mind control"... especially because when hypnosis is used to boost some other psychotherapeutic intervention, it doesn't seem that impressive.
Furthermore, some (again) 2017 research found that hypnotic sugestibility correlated with some psychiatric disorders (lifetime diagnosis) as well as the subjects' current mood:
The passing rate of the SHSS:C ‘Moving hands apart’ item was higher in bipolar I patients than in controls, whereas for ‘Mosquito hallucination’ the rate was lower. The passing rates of ‘Mosquito hallucination’ in controls, ‘Arm rigidity’ in bipolar I, and ‘Age regression’ in bipolar II predicted the respective MDQ [Mood Disorder Questionnaire] scores.
Although published in a reputable-enough journal (BMC Psychiatry), the study was conducted in a Chinese university hospital (although according to Wikipedia it's an "elite" medical shool), so perhaps more research is needed before drawing firm conclusion on associations between mental illness (or mood) and hypnotisability. While the paper did Bonferroni-correct their multiple comparisons, I could not find anything in it about the medication status of the subjects.
Thers's also a 2015 Australian study comparing some schizophrenic experiences with hypnosis:
Passivity phenomena in schizophrenia are characterized by a sense of diminished
agency. [...] Our aim was first to investigate sense of agency
and clinical symptoms in schizophrenia, both classic passivity phenomena and more
general positive symptoms; and second, to contrast the agentive experiences of patients
with a previously tested sample of 370 nonclinical, hypnotized participants. Twenty-six
patients with schizophrenia completed ratings of classic passivity phenomena and of
involuntariness associated with a particular experience of agency alteration. [...] Patients reported considerable levels of involuntariness for both body-related
and thought-related symptoms. Overall involuntariness ratings from patients were
similar to those of high hypnotizable participants in hypnosis. These results indicate
altered sense of agency is associated with a range of experiences in schizophrenia, not
just classic passivity phenomena. Moreover, the experience of altered agency in
schizophrenia was similar to that seen in hypnosis, suggesting that hypnotic analogues
may be a useful way to test theories of passivity-like phenomena.
This probably fits with the theory that most psychiatric disorders lie on a spectrum, with subclinical features seen in the healthy population. The "spectrum of self-agency awareness" (if I may coin a term for this notion) is rarely talked about though... although a quick search found a paper discussing it in autism; no mention of hypnosis in that paper though.
