Good for you! Not everyone lives 10 minutes from an immigration office. I've never had a problem with the app, but there again, I enter the information correctly!
Perhaps the president of the tourism association should be talking to Bangkok Airways who hold the flight monopoly for Samui about dropping their prices, and maybe getting the immigration office to make an extension of stay a 15-minute process (which in reality that's all it is). Then there would be no need to change anything. Just another pie-in-sky dream
Generally it's your passport, a certificate of residence and possibly your lease. Go to the branch and enquire. It seems there's two possibilities at Emquartier so try both
I don't know, I never use agents, I just know from a number of reports relating to opening bank accounts that Thai Visa Centre will only open a bank account if you're also obtaining a visa through them. I don't know about any others