That is a clean looking logo indeed, I like it and could be an nice alternative for people that are looking for other looks.
1. Not for C# specifically no, BUT if you know C# well learning the necessary for Nim is easy, only thing you have to stumble over is the OOP-ness which Nim doesn't enforce. The beginner resources in the "Learn" tab should work fine for you: https://nim-lang.org/learn.html
3. Godot-nim works fine, it uses GDNative. It is usable albeit some bugs need to be ironed out because of Nim 1.0 support. It doesn't support web export. Compared to C# (a lot less edge cases) imho it's on par, however they're making a lot of progress in improving C# support. If you like a code-only workflow try out nimgame2: https://github.com/Vladar4/nimgame2
I tried vim, but extension installation process is horrible or i am not smart enough to do it.