@sunOff-hand, I’d guess that the plugins you tested are all related to listing/viewing comments, but do not enhance or customize the comment form. :)

That sounds like a good explanatory guess ... and it is correct! :)

At least, afaik. MCEComments adds a bar of editing-icons to the top of the form-container, but I could see it doing that, with touching the form.

Indeed, I have not addressed customizing the form-code, or intentionally/knowingly sicced any plugins on it ... but there are changes I'd like to play with, for my theme.

I will have another chance to explore this tomorrow, or maybe there's another experimenter out there...?