For instance, I would like to add some explanatory text on the registration form, to tell the user to ready their mobile device and install some Authenticator app, before they proceed. Primarily because this screen expires so quickly and I do not want to keep TOTP registration open for long.
I would like to be able to change any/all text elements on that screen at will. Including my own translations for those elements.
I would like to be able to add links and/or buttons, re-arrange the controls, add additional logic, for instance using cookies. Or add the dreaded three questions functionality, etc.
It's just so very restrictive the way it is. There dozens of possible reasonable scenarios where that would come in handy, or be necessary.
I think most of those things you list are possible. I'm not sure about the additional logic, but if you know what you are doing with JS then you could implement your own listeners.
I have listed a few specific items, but the main point is that it's far from complete, that list can be endless, hence the request is rather generic.
The bulk of the page is generated and it's outside of our control in Delphi. I cannot even insert any text on that page for starters - if I try, it gets obscured by the generated content. Yes, I can CSS the BODY any way I like, to use an image and place text there, but it's not as good, obviously. I assume a lot of such stuff can be done in JS, but it's a Delphi environment and I don't want to go any deeper into JS than I already have, I need to do this in Delphi. Even a simplest thing, like adding a link on that page would take a lot of JS coding effort for me.