you just don't worry , you can extend your stay a week or two before the February 18th stamp of admitted stay. The requirement about when you need to do a 90-day report should have been stamped in your passport. Check for the paper slip , it should be there. In case you missed or are late, it is just a fine of 800-2000 THB and does not affect your permit of stay. However you should make the trip to the Immigration serving your location ASAP to clear the matter
you need to have a plan for what you will do after December 19. You might be ok to apply for the "unable to fly out" stamp before your existng stamp runs out, which would take you until January 29th
and when you do it online, and insert under "date of end of permitted stay" the date when your extension is finished, your next 90-day report will be on that date, regardless if it is 90 days from now or only 30 days or 18 days
you will need the financial proof for the application to the visa itself, (not for the subsequent Extension of Stay) And SOME immigrations at this point ask for proof that the funds came from abroad. Some other immigrations don't ask for it. This is what you have to expect . . . . . Good Luck!
It is looking bleak. If you haven’t gone back to Phuket Immigration to close out the „under consideration“ stamp, you are technically on overstay since October 12 th. I’d visit Immigration ASAP and hopefully you can apply for a 30 day extension without embassy letter