If you're doing visa on arrival, you need a exiting flight. You can use the website mentioned above, but it's not a real fight. Or book cheap flight to Singapore for $60-$90 dollars
it doesn't matter what you put. You need COE no matter how you plan to enter Thailand before November 1st. If you haven't started COE process yet, then you might need to plan for another departure. Even visa exempt (30 day) needs COE.
If you leave in a week and have not started the COE, your might not be leaving in a week. COE from Washington is average 7-15 days. You might wanna call them repeatedly until you reach someone
The way Thailand is, I'm pretty sure getting your results will get messed up and you'll be stuck in a hotel. Sadly Thailand app or online systems are trash. The amount of people who will be coming over the next 2-3 months is going to really mess up any Thai system they put in place.