Few questions related to programming microcontrollers
The Arduino IDE uses C++ primarily, which it preprocesses in order to make compilable. But you can still for the most part use C in the Arduino IDE and it will just work as normal; there's nothing requiring you to use C++ or the Arduino libraries (or even the Arduino IDE) for most tasks.
Not touching this one. And in the end it doesn't matter terribly since they're very close.
Any of the ATmegaXX8 or ATmega8 chips will fit the socket. Note that they will require separate cores if you're using the IDE. And you can use any chip without the board in the IDE as long as you can find a core for it.
Not if you use a traditional compiled language to program it. If you use a VM then you conceivably could, but you'd have the source on your system regardless.