They do allow walk-ins, in fact they have very few available slots for appointments compared with the total number of applicants they serve. If you can make an appointment, I'd suggest to do it to save yourself some time.
The default time zone shown is based on your system settings, nothing immigration controls. As you wrote, just change it to Bangkok. Yeah, it's pretty nonsensical not to have it hard-coded to Bangkok, but then again, I've seen worse.