You don't need an agent to open a bank account (if you don't have one) when on correct type of visa and applying for the 1 year extension is very simple, especially if you're going for the 800k in the bank method. Why pay for something you don't need?
You're overreacting a bit. With the correct visa and a residence certificate from immigration, you'll most likely succeed in opening a bank account. Don't listen to clueless people and scaremongers.
It depends on bank and branch. They basically have their own requirements at the moment. To make it a bit easier, get a residence certificate from immigration.
I don't see 800k as a lot of money,maybe you do. It's a small part of what I have. You are just another person whining about parking money in a Thai bank like it's the end of the world.
You will be ok with 30 days left of your 90 days Non-O to apply for the extension. 60 days is enough to figure out the financial part and find a place to stay.