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.
How do you not have documentation showing your location? You got a stamp in your passport when you entered Vietnam. Without that your visa is rejected because you are not proving you are in Vietnam.
Which embassy you choose is dependent on where you are. You must physically be inside of the country where that embassy has jurisdiction. For countries that have multiple embassies/consulates, their website will usually tell you what part of the country that consulate has jurisdiction over. This is from the US for example
Every Thai embassy in the world is on the e-visa system and only accepts applications through the online portal. You won't even be allowed onto the embassy property without the printed form that you receive after submitting your application online, which lets you go to the embassy to pay the fee for the visa.