You got a rogue check in agent who didn't know what they were doing. I wasn't checked and I am confident I'll always be able to get in on a one way ticket especially with the DTV, which is even better than the multi entry non-Os and non-Bs I was previously on.
All I can say is, if you possess a Thai work permit OR are in the process of obtaining one and have documentation to show this, then applying for a Thai bank account is easy.
Without one, it's a crapshoot. It then depends on the banks own policy, that of the particular branch or whether you use an agent or not.
It's not a rumor, it will happen eventually but no date has been mentioned. Once the ETA system does go into effect, it will ONLY apply to those who do not have a visa.
While I wouldn't use an agent to extend a visa these days (in Cambodia, Vietnam and Laos, yes, since that's the system there) I would use one to open a bank account if I needed to. Since I already have 2 Thai bank accounts, I personally have no reason to ever use an agent for opening a bank account but I can see the merit in using one.
Never is not true. The only failsafe visa types to be on to open a bank account anywhere in Thailand is a non-B or non-O WITH work permit. Without a work permit, you're at the whims of the policy applying to that bank or branch.
There are several reasons, mainly to do with money laundering. One principal reason cited by Bangkok Bank is due to Burmese citizens using their accounts for nefarious purposes including scams.
Another reason is compliance with international laws; Thai banks are being pressured by the west to comply with certain requirements, one of which is they want foreigners to be resident in the country, rather than just being a tourist.
