There will always be ways to use an agent. Just check with your agent a month out as they said. If they can't do it, find a new agent. It's only Bangkok Bank that's affected by a new internal policy, and that's only if your agent hasn't found a new Bangkok Bank branch that will work with them.
You can apply up until the last day your visa entry stamp is valid. But that's definitely not recommended.
At most offices you can apply for retirement extension when you have 30 days or less left, and some even allow when you have 45 days or less left. The extension will be added to the end of your existing stamp, so there's no reason to wait until the last minute.
You just need to make sure that your 800,000 baht has been in your account for 2 full months already when you apply for your first extension.
Do you have a house or apartment in the US that you'll still have. Or a family member that lives in the US? The best VPN is getting a router that hosts a VPN server setup at your/their house and you connect to that. That way it looks like you're at their house and you'll never have to worry about being detected as a VPN from streaming services.
Correct, there are only 2 financial proofs you can use for the FIRST 1-year extension.
1) embassy certified proof of monthly income
2) proof of 800,000 baht in your Thai bank account
In order to use #2, the money must be in your account for 2 full months already at the time you apply for the extension.
For full details, you need to go to your immigration office, and ask them for their handout for a retirement extension. That will have the specific requirements for your immigration office as they vary from office to office.
It doesn't hurt to ask if you can use monthly transfers, but I've yet to see someone able to do that.