There are half a dozen duty free shops in the baggage claim area, though they're smaller than the ones before immigration, and MUCH smaller than the ones in the departure area - they basically just sell alcohol and some beauty products.
The US affidavit, which is normally required by the amphur. If the OP wants to try to do a free-form affidavit in which he changes the wording so as to avoid misrepresenting himself, he could try that, but it might or might not be accepted.
He also asked "Is getting a US marriage certificate translated and registered...more of a hassle than getting married again in Thailand?" That's the part I was addressing.
To marry in Thailand, the local amphur would require you to submit a notarized affidavit from the US embassy in which you swear under penalty of perjury that you are single. Since you're not single, that would seem to present a pretty major obstacle.
That person was either lying/joking, or doesn't actually work in the visa section (the embassy is a very large place and the vast majority of staff have nothing at all to do with visas). Numbers go up and down, but in most years the approval rate is around 80% or higher.
If by "this issue" you mean young, single Thai women having difficulty getting US visas, it definitely didn't start within "the past couple of years" - the same complaint has been made by American boyfriends throughout the 30 years I've been coming here, and I'm sure it wasn't new even then. Thailand's relationship to China has absolutely nothing to do with any of this.
Decisions like this one are based on visa law and have literally nothing to do with politics - and the US certainly isn't "punishing" Thailand in any way at the moment.
1) Many decisions were made that quickly even 20+ years ago, when officers had no information about the applicant until they walked up to the window, and therefore could not possibly make a decision in advance.
2) The description of what happened during the interview was completely from the applicant's perspective. Applicants are under a lot of stress during the interview, and often don't know or notice what the officer is looking at or considering, or realize how long the interview is taking. It's totally believable that the officer didn't look at her documents - in most cases they're irrelevant - but the interviewer always looks through the passport (not that that would make her feel any better).