it's certainly a risk. But what is the agency's solution? Do they want you to pay them more money to help you get into Thailand? If that's the case it should tell you all you need to know.
Why don't you just apply for the non-O visa from a Thai embassy and then you'll definitely have no problem entering Thailand. And then using an agent for the extension should be significantly cheaper as well. Did they mention that option, or they want to maximize how much you're paying them?
Should be 500 baht. At least it was a few years ago.
Have 500 baht ready with your passport. Don't make it obvious. Just ask for express service. Have a few extra hundred in your purse/wallet in case the price has gone up.
Yes. That's normal for many immigration offices. It's how they verify the address is actually your address. Because it will go to you by mail.
But did you ask for express processing? They usually offer that for 500 baht for next day pickup. But you'll need to be there early and can't be too many people around when you ask for it sometimes.
I wouldn't put all that work into it for a 1 year lease. They're just getting you to do all the work they don't want to do and then after a year they'll kick you out and put the rent at the 55-70k you mentioned.
Unless you can get a lopsided lease where only you can end the lease with notice at the end of each year for at least 3 years, I wouldn't consider it.