What marriage documents are you waiting to arrive? This is not a normal thing in the process. As an American you personally get a notarized right to marry document from the embassy, get this translated into Thai, and approved by the ministry of foreign affairs as being accurate, then with copies of your passport, and her ID card you go get married at the local amphur. There is no waiting for anything. The whole process takes a day, or two, it is that simple. Again, what documents are you waiting for?
As long as you have bt500,000, or the equivalent, in a bank account anywhere in the world in your name there are lots of different ways to get the DTV. If you don't have that amount of cash then forget it.
you are making this too difficult on yourself, and thrown off by what you hear, and what people are saying. You are missing the important detail about why. To make this all very easy apply for your type O visa based on either retirement, or having a Thai wife, at a Thai embassy before you enter Thailand. This gives you the correct visa to get your Thai bank account in your name, transfer your bt800,000 into it, and after it has been in there for two months apply for your one year extension. It is very easy to do. Coming in on a tourist entry of any kind makes it difficult, and expensive.
I get all my insurance, even my health insurance, and accident insurance, through my bank, Bangkok Bank. I trust them, and have never had a problem with it paying
It's not that hard to do. Get your bank statement in the UK showing you have the equivalent of, or more, than bt800,000 in the account in your name only. Copies of your passport, living details in Thailand if not initially using a hotel. There might be a few other UK embassy specific, but your current bank account in the UK is the main thing. This will get you a type O 90 day visa once you enter Thailand, giving you the correct visa to get a Thai bank account in your name, transfer bt800,000+ into it from your UK bank, then after it's been in your account for at least two months you can apply for your one year extension. It is the easiest long term visa extension to get. Just make sure you do the first part in the UK, as without that 90 day type O visa it is almost impossible to get the Thai bank account you need by yourself on any tourist entry, even the DTV.
He does not need any special visa. This comes under the same situation as going to a trade show in Bangkok, meeting clients, buyers, or sellers, then leaving after the show ends, having made sales, or purchase orders.