I stayed at the Centre Point Silom, was very conformable and food was good. good views as well. i paid 50,000b so it was in the middle range of prices.
they shouldn't ask for return flight. since you are allowed to stay for 270 days (with extensions) I came on a TR visa and they didn't ask for my return flight. if they do want an exit ticket you could book on onward ticket for like $10.00. You'll need to update your COE and accommodation booking after quarantine (and ASQ) for the new arrival date and flights.
Yes. The visa is valid for 90 days. Meaning you have to enter Thailand within the 90 days and then your first 90 days in country start when you arrive. Book your flight and asq then apply for your COE.