You're correct... joint account needs double money... period. Also, as Brandon mentioned already, not all offices like/accept joint accounts. Don't listen to your other "experts"... they probably talk about experience when using some kind of (more or less shady) agent...
The day you enter is always "Day 1", so... add 89 days to your entry stamp... that's your "due date". Since you report in person you have a time window of 15 days before and 7 days after the due date. There will be no notification (that's only after you did the report online), it's your job to take care of it.
I'm using facial recognition with Bangkok Bank (activated about two years ago) and also with Kasikorn (new account from this year). Never had any problem...
Yep, even for shared accounts... only one registered person, the app only on one device, phone number has to be registered with the account owner. If you want to change your phone or tablet or whatever you use, you need to go to the bank. That's how it is, at least for Bangkok Bank.