Not now. The visa once issued would be valid for 3 months from issue. Apply about a month before you leave. London has a pretty quick turn around time.
Get a letter from the hospital doctor that you can’t fly and can’t attend immigration. Ask a friend or someone from the hospital to go and get the normal 30 day extension first.
Then when that’s running down repeat to apply for a medical extension. Depending on what the doctors letter says they can be issued for up to 90 days.
What you’re referring to as a “marriage visa” is actually a year long extension of stay. You can only get that for the same reason you got the Non O. You can’t get a year long extension off a tourist visa or exempt entry so you have to switch to a Non O and then apply for an extension.
As you are applying for a Non O based on being married to a Thai national then they are asking for two witnesses.
As to making sense. You’re looking in the wrong place. TiT.
Each office will use its own rules regarding combination funding. Perhaps someone from Trang is in Thai group and can advise. If not post in a few Trang related groups and see someone there knows.
Failing that you’ll have to slog out to the immigration office and ask them their policy.