1. I don't know what ACS is so I'll skip that. I also don't know what you define as a "complete mod" but QuakeC lets you create entire new monsters, define all their animation frames and what code happens on those frames, etc.
2. It has lots of bells and whistles added, but everyone hates a mod that requires them to use an engine other than their own favorite. Nearly all quake mods (at least the ones you'll find on this forum) are as engine-agnostic as possible, sticking only to what features the majority of engine ports have all added in a roughly standard way (raised map limits, fog).
3. InsideQC is the best. Second best is just reading through the coding help and mapping help threads on this forum, the good quake wiki:
and Preach's blog which is full of crazy hax:
4. putting 'qcc.exe' and 'quake.exe' in the same batch file is about as fast as it gets. what are you hoping for, live editing of game code?
6. You can't concatenate strings or use arrays, and it's not OO. You can abuse globals and create mindbending bugs though. You also can't touch whatever the engine does, so you can move entities within the existing physics system but not write a new one, etc. If there's an engine effect that doesn't have a quakeC builtin function you can always abuse 'stuffcmd()' which does it through the console instead.
7. I don't know, I guess nobody really plays Hexen 2? Nobody played Arma2 until someone made this crazy sick zombie mod for it and suddenly everyone had to buy Arma2 just to try it, so, if you truly believe in Hexen2 and/or yourself just do it anyway and pimp it and if it looks good enough people will want to try it.