Good to see you are interested in doing your homework before embarking on what will probably be the most intense (albeit brief) drug experience you will ever undergo. Anyhow, here's my 2 cents…
1A: Dosages vary depending on a myriad of factors, including the desired effects (of course), set and setting, individual tolerance to the substance. Some people seem to be more sensitive to DMT's effects than others. Don't rely too heavily on published data pertaining to DMT's dosage. What may be considered the amount necessary for threshold effects could be inactive in some, while overpowering in others. Try smoking the smallest amount possible, and gradually build up the dosage until you feel sufficiently DMT conscious. If you want to experience a longer lasting, milder DMT experience, I would recommend Ayahuasca/Pharmahuasca. Both substances are non-DEA scheduled, and are available via numerous mail order companies.
2A: DMT, as far as I know, is inactive when any method other than smoking is used. DMT is only orally active in the presence of an MAOI which inhibits the metabolism of the drug. I would assume the same holds true for snorting it also. I've never heard of injecting aqueous DMT, so I wouldn't know the efficacy of doing so. Your best bet in the case of pure DMT is to puff on the stuff.
3A: This is tricky. The DMT experience tends to be very enveloping, and it seems to overpower the effects of most drugs. Being high on marijuana (at least for me) makes the switchover from reality to DMT hyperspace less disorienting. Smoking marijuana as the trip subsides will also bring back some of the effects of the drug (mainly boldening of fading visuals; mind effects such as "DMT consciousness" are notably absent, at least in my case). The only other drug I've tried with DMT other than weed is salvia divinorum. Not a good idea, as I experienced amnesia. I regained "normal" consciousness sometime later, bong in hand, knowing something fucked up just happened, but for the life of me I couldn't remember what. Amyl Nitrate or Nitrous Oxide tends to intensify the effects of other hallucinogens, although I've never tried it with DMT. I can't really imagine the DMT experience being more intense than it is.
4A: Yes indeedy. Go to any health food store, or OTC section of a drugstore pharmacy and look for capsuled Yohimbe; it is a naturally occuring MAOI. If your peer group is mostly drug people (like mine is), there is bound to be at least one person you know, or that one of your friends know, that has a perscription for Prozac or Zoloft (depression seems to be a sort of drughead plague). Just ask for 1 or 2 pills, as a healthy person may notice many unwanted mental and sexual side effects with such drugs, although they are more frequent with regular use.
5A: Personally I enjoy outdoors. It's awe inspiring to see nature's majesty fractalizing into a spiraling infinity.
6A: For me, I always like to trip with people around. The collective energy of 3 or more people in DMT hyperspace is truly amazing!
7A: Hard to say really, as the only times I've seen people high on DMT is when I've been under its spell myself. Gaping mouth and/or dopey smile, blank stare and immobility seem to be the norm from what I've noticed.
Hope this was helpful. Good luck, and happy voyage!