The semantic N400 and syntactic P600 are some of the most well established ERP components in the literature. They were discovered decades ago and the results have been replicated hundreds of times in other papers. There is just no question that they are present and very robust. That's not saying they are there 100% of the time in 100% of people. All we know is that it's a very easy response to get out and by now it would be surprising / worthy of investigation if people didn't show them in the standard paradigm.
There is a paper from a few years ago that looks at the 30 years of N400 research and all the papers that found the effects. It's probably a bit out of date now but it does a great summary of the research up until that point.
I think Hillyard & Kutas (1983) is the original paper.
ERP components are not always so tightly linked in time. You can module them based on task context and shift them by all sorts of ways so it's never that the peak is exactly at 400 ms post-stimulus. It can start WAY before then and end WAY after then. Broadly, we use N400 to talk about a large response that peaks "in that time window" (so, from 300-500ms). I wouldn't get too fixated on the specific milliseconds. Therefore you don't "use" the amplitude for anything (not sure I understood that part of your question).
Typically you see N400 over central-parietal electrodes. There is a bit of confusion over exactly what the cortical sources are but MEG and fMRI do support the idea it's a strong dipole with the positive component slightly in the LH and therefore EEG electrodes over central and parietal sites pick up the strongest signal. This also gets at what you use as your reference electrode because EEG potentials are differential and the patterns you see on the head are very dependent on reference / ground settings.
Typically you would group electrodes over general position and look at central-parietal ones or you could plot individual responses (both ways would still show the N400). As with anything in science, it's up to you what you show.