Sit down and negotiate, it's true (especially post covid) that people are skint. Invite her for a sit down and ask her to do a pre-inspection/indicate what (if any penalties) may be incurred. Be open about your concerns about the sofa and maybe you can come to an agreement rather than leave it to the last moment. Take a Thai friend or translator if her English isn't good enough.
There's absolutely no need to switch the reason for your extension - marriage to working. Perfectly ok to teach/hold a WP whilst on a marriage extension.
Straight to the airport. Don't get into trouble /arrested beforehand. Make sure you have the cash ready when you arrive at the airport immigration. Cash, apologetic behaviour and you should be fine.
Charge inventing is inevitable :) I know what you mean though. 1000 baht will feed a Thai family for a few days or so. Just chalk it up to experience. Always be on good terms with your employer here when you leave, visa transfer, work permits and an employer reference will still need to be done.
Not really, the clue is in the name, you are exempt from needing/using a visa, so they put a stamp in your passport, a no-need for a visa, visa exempt stamp.