you can change the address on your account. I just did that with my Kasikorn account. It shows my new address on the statement but it still refers to the home account branch where I opened it. So there's no need to open a new account. The only thing I am lumbered with is a fee when I use my ATM card because it's not in the home province. So I avoid that by doing cardless ATM withdrawals. No big deal, but it's a bit of a weird rule in Thailand.
my non O has a one-year extension for retirement so I guess that's why I have been able to stay here for a number of years because it can be renewed here within Thailand and it requires only 90-day reports without having to leave.
a multi entry visa allows you to leave and return to Thailand as many times as you want which extends a one year visa to 15 months. You don't have to go to Ranong every 3 months you can stay in Thailand for one full year if you want without leaving. You just have to report to immigration every 90 days to do a report which takes 5 minutes.
there's nothing stopping him from visiting that Burmese Island every 3 months if he wants to. He will get stamped back in with his multiple entry visa. He must really like that Island.
if the expiry is one year after it was issued that's when you have to leave. But in the meantime you will have to do 90 day reports at your local immigration office.
if you have a multiple entry as an addition to your non O you can of course go in and out as much as you want. But you don't have to. I have a non O and I haven't been out of the country since 2015.