New bank reporting rules introduced in April this year has been a very big clamp. Over the years I've had about six different accounts, never been a problem. A couple of months ago I wanted to move my branch to another branch (both within Bangkok), but caused too many issues so I just left it as it was. Certainly not as easy as it used to be
You can't convert a visa to Multi-Entry. All you can do is get a re-entry permit which allows you to re-enter on your current entry stamp. This can be done at the airport, takes ten minutes, better than traipsing to an immigration office to do it
Samut Prakan is well known for being very independent and anal. Simply go the office and ask for their list of requirements. It doesn't get much easier than that
Worse case scenario is you're denied entry. Best case scenario is you're stamped in without an eyelid being batted. Immigration tend to look closer at perpetual back to back entries. Use a visa transport company