(Joking aside, I actually hope great cooperation, not competition... it's what open source is for. Seeing someone else is making a .Net wrapper as well is just plain awesome and I wish you the best. I really like the different API style.)
As a stretch goal, you might try to integrate https://github.com/magiblot/turbo too. This is a Scintilla-based text editor control implemented in tvision. We worked together to make it embeddable in other programs (because I wanted it in mine). Example usage: https://github.com/tmbasic/tmbasic/blob/master/src/tmbasic/C...
Also, I assume you know about the Turbo Vision Pascal and C++ books. They're really helpful. I transcribed both books to Markdown for easy searching if you want it.
That editor is really impressive. I'll see if I can wrap it too! Can I ask what your editor integration is for, what project?
I know the books exist, but I haven't read them. It's possible I even had a copy as a child - we had many of them but I never wrote Turbo Vision code, just Turbo Pascal in plain text or graphics modes. Then I moved on to Delphi. I would love to find both in MD - are they publicly available?
I realised I never answered your question about P/Invoke: I wrote (as in got an AI to generate) a flat C wrapper using the handle pattern, and then reimplemented classes back in Oxygene Pascal. I'm experimenting with new controls now, eg a tab/page view and others. Each is just a class that has its own view, plus some management extras, such as what max rendering it supports (I'm adding DOS character sets through to Powerline in rendering tabs, borders, etc.) It's all churning quite a bit as I do something, realise it may not be the best approach matching the TV model, rethink - learning a lot about TV's architecture at the time - so no more solid answer than that currently simply because it may be outdated tomorrow :)
Rather than focusing heavily on inheritance, I'm leaning more towards soft interfaces (duck typing) and wrapping via composition. This is just personal preference. But the concept is, if something looks like it is able to hold controls, it is treated as though it can hold controls, for example.
I've sent you an email with the Turbo Vision books in Markdown, if you didn't receive it, hit me up via the email in my profile.