1. The drywall and green glue process is really simple, so any good dry wall guy can do it...what is important is that you use 3 tubes per 4' x 8' sheet and apply like spaghetti on a plate...so when installed you dont have void sizes greater than a quarter. Also try to keep a minimum border around perimeter, they say 2" but this is too much....1/2" or so is better....
2. We have a lot of clients who purchase direct and have a contractor install...
3. It would only add a little more time to what it would take to hang drywall...