I have never been in this situation myself, but I have a couple of thoughts:
- I wouldn't think the missing departure card itself is an issue - I have seen them handing cards on BKK to tourists who lost theirs.
- my feeling is that the letter from your embassy is just over the top. BKK customs might not even understand what it says, but since it is not addressed to them, they might have decided that the least risky for them is to send you back to Chaeng Wattana.
Without criminal records, at BKK immigration they basically do not care how long you overstayed, just pay up and they will let you go.
Since you have an old passport showing a formal entry stamp, and a new passoprt that is valid, if I were you I would show up at the airport nicely dressed and shaven, with the 20,000 baht and the two passports. And well on time, since the procedures might take some time and you don't want to miss your flight.
I would not say anything if they didnt ask about it, and if they asked about the reasons for the US to revoke your passport I would tell them there was a small issue with your ex-wife that since then has been cleared. If the immigration officer is a man. If the officer is a female, that explanation doesn't go well, so you might change it into tax issues.
Play cool. Do not be nervous. Imagine they are all naked - that will keep you focused on something else than your nerves.