when you start filling in the thailandpass you just need to follow the instructions of what to do when they ask for hotels etc, very simple and instructions easy
sealed flights will only show up in availability if booked as a direct connection from an intl inbound flight to bkk,,,,so just looking at bkk hkt will not be enough,,,
you need to have a complete ticket from origin-bkk-cambodia, it should not be separate tickets,,,as bags need to be tagged to final destination and you need to remain in the transit area without going thru any immigration or customs areas.
Unless Thai has totally different rules than USA, then she should be arriving into the Kingdom with the Thai PP for her. I would certainly check with the Thai Embassy on that.
there are a lot of requirements to do before you can even attempt to fly to BKK via the Thailand Pass that must applied for, good thailand pass and go thru that process, which will also require quarantine in BKK as of right now.