#2
Well, you *can* record the output of the headphone jack to whatever input you want on your interface. The question is whether or not it will sound any good. It will probably sound okay for doing recordings for the purposes of evaluating your playing, letting a friend hear some licks you're working on, etc. It will almost assuredly not sound good for recording an album that you are going to release for sale. It kind of depends on your expectations for it.

The only way to tell is to try it out. You certainly won't break anything by trying.

CT
Could I get some more talent in the monitors, please?

I know it sounds crazy, but try to learn to inhale your voice. www.thebelcantotechnique.com

Chris is the king of relating music things to other objects in real life.
#3
The best thing to do is bypass the can of angry bees that is the MG and go guitar->instrument input on the interface. Then you should look at some guitar modeling software, maybe even freeware like Lepou Lecto and IR loader like Lecab will do a lot better than what's in the MG.

Otherwise - look up cheap dynamic Behringer mics if on budget
http://www.zzounds.com/item--SAMR11
or just go like the pros do it with this one
http://www.zzounds.com/item--SENE835