I am doubtful you're here on any visa (that you bought from a thai consulate before you winged your way here) and it's far more likely you just showed up and got stamped in free for 60 days on the visa exempt entry program. 🙂
Be that as it may
If your stamp runs out on a day the immigration office is closed (weekends or holidays) and you go the very next day the office is open (in this case Monday Dec 8th) you can get your extension like normal for the 1900baht, will not be charged for overstay but you will get an overstay stamp in your passport that says "overstay xxx days fine waived due to closure"
Just a tip, there's no need to run your stamp down to the end to get an extension, most offices (other than Jomtien/Samui) allow you to apply for an extension when you have 30 days or less left on your current stamp and you don't lose any days applying early as the extension is added to your current expiration date
NO the process doesn't take longer than the under review period, IF you don't have the required minimum stamp time left they won't let you apply, and if they think you won't get the visa, they won't let you apply
I have never seen someone apply, then go back in 2 weeks and be denied the Non-Imm
no, you need to have the required days left on your current stamp when you apply for the Non-O visa to cover the 'under review' period
In Bangkok you need a minimum of 15 days to apply for an in country Non-Imm visa because it goes under review for 2 weeks. After you file the application for visa/pay the 2000baht fee they give you a receipt and it has a date of 2 weeks out stamped on when you come back
BUT
You're staying here on your current visa exempt entry.
They won't even let you apply if you don't have enough time left to cover the under review period
And in Chiang Mai you need a minimum of 15 BUSINESS DAYS (about 21 calendar days) to apply for Non-Imm visas up there
You can file a 90 day report in person from 2 weeks before your due date until 1 week after the due date (giving you a 3 week window to report)
SO
if you had a 90 day report due on Dec 3rd, your "window of opportunity" to apply without a 2000baht failure to file on time fine is from Nov 19 until Dec 10th
As long as you show up on/by the 10th of Dec to file you're okay and will not get a fine for failure to file on time. <- That would happen on the 11th 🙂
While it's true if you're coming here "free stamp" (60 day visa exempt) thai officers at passport control CAN ask for the "three proofz"
*proof of onward travel within the days you'll stamp in for
*proof of lodging
*proof of funds (which is 20K baht or the equivalent in CASH)
the likelihood of being asked for ANYTHING is slim to none
Unless you're trying to live here on free stamp entries (in which case they'll ask for proof you're going back to your country in 60 days) there is very little chance of being asked anything when you stamp in
Most "real tourists", aren't asked anything when they stamp in.
Coming in free stamp you have a much higher chance of being asked for onward travel by your air carrier rather than by thai officers at passport control