A non-OA visa is only available from your home country Thai embassy. Are you sure you're correctly talking about a non-OA visa? This is a visa that requires mandatory health insurance every year, a criminal records check, a medical certificate, and other things.
The non-O visa does not require any of these things and is available from almost any Thai embassy and from inside of Thailand if you already have a Thai bank account.
I'm not sure what you are asking. There are multiple sections to upload information. If you need to upload more than one document for a section, use an online tool to combine documents into one file.
Worst case scenario, you only lose one application fee. That's because your husband has to apply and receive his DTV before anyone else can apply. One of the required items to apply for a dependent DTV is the already issued DTV of the primary person.
For the US Consulates and Embassy, they usually want to see 3 months of bank statements, but only care what the balance is on the most recent one.
That's plenty of time to get your visa. London is usually pretty good.
Worst case scenario is you fly without the visa and hope it comes while you're traveling, or enter visa exempt and just use the visa next time. Unless you have a long history in Thailand this year, and you're worried you won't get in without the visa.
If that's the case, you can use a safe entry agent to get in this time, and then try the visa the next times. That is probably cheaper than changing a flight.