If you want to move to the income method after the first year (you need 800,000 in the Thai bank for your first retirement visa), you can deposit 65,000 per month as well as the 800k. That qualifies you to get the extension the following year and you can then spend the 800k.
You do not need to show proof of income, it is all about money in a Thai bank account.
You still need insurance if you apply for the retirement visa in the UK, so most people enter Thailand on a tourist visa and apply here.
Never tried for a marriage visa, but anecdotally it seems much more troublesome than the retirement one.
to be honest Brandon I have never encountered that problem here, but I think your logic is what I would assume also. This is Thailand, so we might both be wrong!