Bumping this older thread while searching for a friend about passport validity and extension of stay.
From the official source that airlines use (TIMATIC) it is clear that there is NO 6 month validity rule for entering Thailand. At least for the example that I used:
From Germany to Thailand, Lufthansa non stop, arrival 15 Dec, stay 29 days, vacation, have return ticket, German national with normal German passport valid until 15 January.
Check on your own with your travel itinerary.
There might be variations if having stop overs or other factors?
https://www.copaair.com/en-cr/timatic/
Query:
(click on images for full size, "birthday" is arbitrary)
Result:
Now cutting short the passport validity shorter than the 29 days intended stay: