Both Hertz and Budget at Suvarnabhumi provided me a great service. However Hertz has small cars only for this year. Not sure it's the car you want to drive to Isaan
Dunno about now, but 6 or 7 years ago Qatar and Etihad always asked for a return flight at check-in. Buying a low cost flight to Siem Reap or Vientiane on the fly usually did the trick. Never been asked for a return flight with a Non-O instead
Theoretically not allowed if you don't have a long stay Visa. Allowed tho, if you have a work permit or can provide a residence certificate by immigration or a residence affidavit by your embassy (if they do).