your assumptions seem ok, but i see trouble ahead, much of which you have already spotted in your assumptions.
As you said, you can't really export c++ classes and methods, you will need to provide a function based c interface. What ever facade you build around that, it will remain a function based interface at heart.
The basic problem i see with that is that people choose a specific language and its runtime because their way of thinking (functional or object oriented) or the problem they address (web programming, database,...) corresponds to that language in some way or other.
A library implemented in c will probably never feel like the libraries they are used to, unless they program in c themselves.
Personally, I would always prefer a library that "feels like python" when I use python, and one that feels like java when I do Java EE, even though I know c and c++.
So your effort might be of little actual use (other than your gain in experience), because people will probably want to stick with their mindset, and rather re-implement the functionality than use a library that does the job, but does not fit.
I also fear the desired portability will seriously hamper development. Just think of the infinite build settings needed, and tests for that. I have worked on a project that tried to maintain compatibility for 5 operating systems (all posix-like, but still) and about 10 compilers, the builds were a nightmare to test and maintain.
