What is the correct start date for my 90-day report after receiving a NON-O visa in Thailand?

You`re getting a lot of «alternative facts» here, as always a lot of folks here more than willing to share what they think in lieu of actually knowing the facts. I have no idea why you were «stamped in» on Nov. 25th. - you might want to check that again with IO. What is factual: Your last visa (if you have had several or even visa-excempt) is the only valid stamp. Non O lasts 90 days and therefore no 90-day reporting. This reporting starts 90 days after you have received your one-year extension. Our western logic does not always follow Thai logic, but on this they are comparable. Good luck!

90 days from 17th Sept

Your 90 report is based on your arrival date in Thailand or your previous 90 say report. Visits to immigration that do not involve 90 day reporting do not count as a 90 day report. In the eyes of immigration the 90 day report is a stand alone document

If you applied in country for your non-o then all the timers start at the stamp date of that visa. Your previous visa is now invalid. Since you would either need to apply for your one year extension within 90 days, there is no 90day report due till after you get your extension.

A Non-O visa should be good for 90 days from the point you get it. They've made a mistake with the date stamp so go back and get it corrected. As to 90 days report when you get your 12 month extension they should put a slip of paper in your passport with the date you need to report

90 days from the 1st day that you enter.

Is that an UNder Consideration stamp?

None. You don't start the timer until you're already on the 1-year extension since that's the first stamp you'll receive that can go over 90 days.