it's 1900baht and you should never leave it until the day before as there could be a problem or multiple problems that you need to fix. Most offices allow you to go with 30 days left.
you need 3 years unbroken visa or 1 year if married with a child and 3 years of continuous tax payments. If you decide to go back to a non-o for citizenship purposes you will reset your clock back to 0. If your long term plan is to apply for citizenship then you are better off not changing over to non-b just to change back to a non-o.
You will need give more information if you want serious informative answers. No one can really answer this based on what little information you've given.
my TM.30 matched my address, and my online report was still denied. Like a lot of things in Thailand, sometimes it just comes down to luck of the draw too.
International driving permit if you're here short term, if you're going to be here long term then you will need a Thai licence.
Which is relatively straightforward just apply when you get here but make sure you still have an IDP. You will need a medical certificate from a doctor here and you will have to take a few very simple tests, once done you'll be given a 2 year permit.
Depending on your passport depends on your options but the only 90 day visa you can get is a Non-immigration visa such as non-b (working) non-o (retirement, marriage etc) or non-ed which all have requirements for you to meet.
You can get a 60 day Tourist Visa and extend it here by 30 days to getting your 90 days though.