I always feel a little sad when I see a falang wife with falang husband in Pattaya. In many cases they've been dragged here and probably haven't been told the full story
Those reminders can be wrong, I had a couple, now corrected. I'd ignore the TM30, too late now anyway as they will fine you circ 2k whenever you go, if in deed due. As stated above clock starts on arrival
Have you tried logging into your HMRC account? Usually you use your tax ID. May show more info including tax code, I'd expect so.
The tax code change is because (apart from the part you mention) your income is greater than the personal allowance so need to pay tax via tax code? So not just basic pension