Certainly not the case, some branches maybe more difficult than others but on the whole it shouldnt be a problem. I changed some Sterling to Baht on 2 occasions at the same SCB branch and the 3rd occasion they asked ME if I wanted to open an account. That was on a Non Imm visa with NO work permit.
The 800k needs to be seasoned for 2 months for the yearly extention of stay NOT the visa. If you're not here yet get the Non Imm O in your own Country, if you are already here on a Tourist visa then yes you can open a bank account and apply for the Non Imm O visa here.
The letter from Bangkok Bank is a standard template that confirms your bank balance on the day. Your bank book confirms the money has been in there for 2 months. I dont know about other banks but BB should know what you require. Cost was 100bt last time I got one.
It's ideal for long stay tourists if they already know where they're going to stay. The accomodation and insurance bookings only need to be made in 90 day increments.