99.9% of people adopting the new version are not going to know what is in that new library or how it operates.
I lack the technical knowledge to give you an answer to all your other questions, but this sentence caught my attention.
I just would like to say this:
99,9% of the people already have to believe every other piece of code of bitcoin because they (me too) lack the skills to review it themselves.
This, in my opinion, is one of the biggest hurdles for bitcoin technology to overcome.
C++ is hard to read, much harder
1 than C, but not impossible. I have a hell of a time with it myself, but I've walked people into the code so that they could personally verify some issues that they were concerned with.
Checking that the code does what it says is much easier than checking that the math does what it should. Large integer multiplication, for example, is not implemented in a straightforward way. If you've never seen it before, you have some (math) homework to do before you trust it. Same with modular fields. Same with discrete elliptic curves.
Anyone looking for a project? A guide, with references and links to examples and tutorials, that takes the reader through the background material and into the code would be fantastic.
1 C++ has much more
implicit behavior than C. For example, in C, you always know when a function was called, and which one. A grep will get you there every time. In C++, it isn't always so simple for the layman.