You can not apply until after you arrived in country, as you will need to upload your Vietnam entry stamp. Furthermore, next week is the Thai New Year. Embassies will closed and when open, have skeleton crew. You might end up having to wait longer because of that.
Typically it is best to have the letter state you have permission to work remotely from Thailand, but you can try with something generic. I am sure it has worked for some and maybe not for others. All depends on the reviewer
how did you upload all that? Did you create one PDF or multiple files? You can only upload one document per section. If you were uploading multiple documents back to back every new document uploaded would erase/overwrite any document you uploaded before it.
Anonymous participant - So the visa agent is probably doing something sketchy. They want to submit your application via the embassy in your home country and use “utility bills from the last 3’months” as evidence that you are still in your home country. The point of this evidence is to prove that you are outside of Thailand and within the jurisdiction of the embassy/consulate you are applying via. If you are traveling to HCMC, you would apply online via the consulate in HCMC. They don’t care about utilities in the US. What you are will upload as evidence you are in Vietnam is your Vietnam immigration entry stamp and your hotel booking.
Yes, the HK embassy only accepts applicants from long term residents of Hong Kong and Macau. It is clearly stated on their website. If you live and work there, you should be fine with submitting your permit and ID.