, most likely you wouldn't be fined unless the current TM30 receipt is expired (with past check-out) date. The rule is TM30 submission within 24 hours after arriving THAT address, not in Thailand. You can submit it again anytime you want/need to.
If you have a valid TM30 receipt already when you have any application at the immigration office, you would be fine. (unless you encounter any police-involved case)
Your 90 days count is stop when you leave and restart when you enter Thailand again on arrival date as day 1. Your next 90-day report due under your situation is 90 days after your latest entry date.
Yes, that is correct. It's a 90-day entry stamp on a visa waiver (not visa-exempt, it's based on a bilateral agreement) and it's not eligible for tourism extensions. You have until 23:59 on 5 January 2025 to leave Thailand.
Q1. Do I need to prove that the funds came from on overseas source?
-> Funds for a 1-year extension don't require international transfer.
Q2. Is there a minimum balance required in my Thai bank account throughout the year?
-> No minimum balance throughout the year other than seasoning. Your balance can be even zero after you get an extension.
Q. As in, what if I temporarily go below 400k baht at some point during the year, so long as it isn’t within the 3 months before renewal; is this a problem?
-> It wouldn't be an issue. However, better to ask your office along with the application for an extension whether they want you to bring the updated passbook when you go back to the office after the under-consideration period.
90-day reporting window in-person is 15 days in advance until 7 days after the due date, including holidays. It includes the time of "acceptable for you to do late a 90-day report". Sadly, you already missed this period. Now, when you go to the immigration office for a 90-day report in person, they usually charge you 2,000 baht for lack of report. This fine is fixed, and won't be changed if you go tomorrow or months later.
The online reporting window is between 15 days in advance and 7 days in advance of the due date, never been after the due date. If you did today, it would be rejected.