no official time. But every entry is at the discretion of the immigration officer, and you can't expect to keep using visa exempt entries or tourist visas as a way to live in Thailand long term. Even spending several months away isn't enough to ensure entry, as your previous stay and entry history will all be considered.
no, but you asked if there are banks that allow this. I believe most do, it not all, from my experience. The person at K Bank might have told you incorrect info, or there are specific fixed deposits that always roll over unless you go in to stop it, and maybe they were telling you about one that specifically had such a restriction.
Never had such a problem. You simply select at the time of opening the fixed deposit whether the funds go back to your main account at maturity, or if you want the fixed deposit to roll into their standard term.
The extensions are routinely done all day long at immigration offices, and are probably the most common thing they deal with hundreds of times a day in tourist towns. From your questions in comments, you seem overly concerned when there's no need to be.
7-11s serve the same purpose. Every 7-11 loves coins. Count them into bags of 100 baht and use them to pay for things at 7-11. The clerks will be glad to accept the coins.