1) Well if you stick the mic dead in front of it, have the mic gain set to unity and turn the amp up/down until it's at the right level.
Failing that, if it sounds good to you then it's fine.

2) If it's an acoustic then I would mic the guitar itself, pan that right a little and then run a direct line from the guitar and pan that left. Just watch for phase.
Of course it's an electric you use, I bet?

3) Drums are to taste. I'd suggest backup vocals go to a side, or possibly split to both sides if you have more than one track of them. Most of it is mix dependent however.

4) General advice is to go easy on the comp with vocals. Better to "ride the faders" and draw a volume envelope (although this takes time).

1- Amps only need to be as loud as necessary to get the sound you're looking for. Get it right at the source before setting up the mic. Why go louder or softer than what is needed?

2- i'm not sure I understand this question.
Are you trying to capture the room? Use reverb if the room isn't providing what you're looking for.
Are you looking for a richer guitar sound. Better gtr, strings, amps, mics, pres, and signal chain will give you a fuller sound

3- All these items can go anywhere. I usually try for a balance. Maybe rhy gtr on the left and keypads on the right.
Keep similar sounds seperate for clarity and distinction, keep them together for an ensemble effect.
Stereo cymbals would normally be panned in stereo- Hard left and right.

4- For typical compression (to keep even levels) Thresholds are set so that little or no compression takes place during the average levels of the song. Compression only takes place when the levels exceed the norm.
Type of instrument doesn't matter.
how much, is usally, as much as possible before the sound begins to sound unnatural.
For special fx compression, the sky's the limit.

Thanks!!!

For the question number 2 about the guitar. I was meaning just getting a wider stereo image so that I can fit vox / solo instruments (i.e. more important stuff) in the middle and make it not sound cluttered...

Is your guitarist up to doubling the track (playing identically as possible on two takes) so you can pan one L and the other R? Otherwise, try duplicating the track and panning L/R with a short delay on one side. If your DAW will allow it, you can put random low-frequency modulation on the delay time to simulate double tracking. We're only talking milliseconds here, 10 max.

Yeah, he's pretty much up to anything. There's just some parts (i.e. keyboard solos) I don't know where's the best place to set the guitar. But I like the delay idea. I'm trying to do something like the beginning guitar in the song "I ready, I am" by The Format.

when do I have to worry about phase?

If it's a keyboard solo, you shouldn't need to put the guitar anywhere :)

Maybe pan it off and almost mute it?

As to your first question, "how loud should the guitar be?"
It sounds like you will be recording guitar and drums at the same time, in which case a loud guitar bleeding into the drums mics can quite possibly give you a "bigger" gtr. sound.
WARNING!!! The problem is, if gtr. player decides to re-record the part, then you have all of that old gtr. in the drum tracks, which is a real problem if he decided to re-write the part and play something different!

Be careful not to record the guitar parts too quiet, or you end up with drums bleeding into the guitar mic, which is almost always NOT good.

Have fun
MarkG