1. There is no real limitation for an isometric game in C2/C3 but it is more complicated than a regular 2d sidescroller.
2. Yes, you could do Hotline Miami in C2/C3. There is nothing in that game that C2/C3 can't do and would be pretty easy to make a perfect clone of if you know how to work in this engine.
3. It depends on what server you use. All providers have their own limits. I use photon for all my multiplayer stuff.
4. Yes you can, but there is no basic plugin for that so you'll have to make your own system and optimize it.