1. No - have to buy before you get there. Can make that transfer easily, even with 2 hours less. Have to go through immigration and pay THB 500 though.
2. I think you will have to go back through BKK and it's one coupon per flight, unless it's the same flight no.
4 . 3 days in REP is good. Better if you can do 7am-5pm every day.