You're making assumptions... They could refuse to let you board the flight. And yes, immigration can stop you from entering the country for a number of reasons and send you back to your point of departure. That said, I wouldn't be surprised if there was a small grace period as this is extremely last minute.
I think this is a temporary measure and a bit of a knee jerk reaction. And yes, it's definitely being applied to seduce what use to be Thailand's number one client market. It's simple logic really: most regular travelers got vaxxed, as traveling supersedes any other concerns for most of them.
Baht and dollars for the visa and most things in Poipet (restaurants, hotels, cafe's, etc)... Some large shops accept baht in Siem Reap, but they're far and few, and it's tiresome to ask as it pisses cashiers off. When paying with dollars, be very careful not to be given damaged bills with your change. This is often the case in bars.