I did it in Jomtien. Need residence certificate and copies of relevant pages in passport. Also plan on having to go back a few times...
I would suggest going to one of the inspection stations to do it rather than the main office. No long lines, they handle everything, can get the insurance there also. For me it was 30 minutes closer drive ad well.
No need to change anything. The only time this could become a cause for question is if you want to do an in-country extension rather than a border run.
Officially not married so not a spouse. So both of you would need a separate application and qualify on their own.
I don't quite understand your comment on the bank account either... The bank account doesn't need to be in Thailand. Your partner couldn't open a bank in their home country or other country where you are?
Either way, to qualify each applicant has to be able to show a bank account in their name with 500k baht. Supposedly some say there is a way for a "sponsor letter" but haven't seen any reports of it actually being done
TM30 is a report made by the property to immigration where you're staying in Thailand.
Then you need to do a 90 day report to immigration if you've stayed in Thailand without leaving for 90 days. It's basically and update so they know where you are. Very simple