What is the process for American expats in Thailand to request a visa extension due to inability to return home?

Mostly successful extension. Two copies of: the extension application, the lease to my apartment, all relevant passport pages, the letter from the U.S. embassy, the email from the embassy to which the letter was attached, signed “change of reason” for stay in Thailand forms, declarations that I think Covid’s too dangerous to fly home and that I want to apply for a second year of learning Thai. I have to check in again in two weeks (I hope not pay another 1900B) and presumably get to stay through most of Oct. I expected the 30-day extension to start on the 26th, and maybe I’ve been short-changed on that, but it does feel good to be fully legal, even it it comes in smaller and smaller pieces. It’s not easy, and it’d be odd to be punished for not waiting until the last week of the amnesty.

First attempt failed, as immigration asked for two copies of the email from the embassy...the letter itself seemed like an afterthought. Will try again later this week.

Yeah, got it in about half a day today, now we'll see what Immigration says.