1. The AWA doesn't factor into your composite score and it's rarely a major factor in admissions... but it can be a minor factor, depending on the program and the applicant. Don't herniate any brain cells on your essay, but you probably don't want to completely skip it, either. Just take it easy during the 30 minutes, and you'll be OK.
2. The IR is also completely separate from your overall composite GMAT score. Like the AWA, it isn't a major factor in admissions yet, but you want to put in a moderate amount of effort to avoid embarrassment, especially if you're looking ahead to next admissions season.
3. The GMATPrep is always the best indicator of your performance. Nothing else is really close. Most people feel that the MGMAT is harder, but if you've spent a lot of time with MGMAT materials, you might actually perform better on the MGMAT tests, since you're used to their question style. Take all non-official practice tests with a grain of salt.
3a. Wait... you're scoring what?!? If you're already scoring 760-790 on the GMAT, you don't need our help.
Or was that a hypothetical?
4. GMAC has been very careful not to tell anybody how many questions are experimental. Maybe somebody else has some secret information about this...?
Good luck with your studies, Posse!
Helping students kick the GMAT in the nuts since 2002... www.gmatninja.com.