@Roland ********
Thai immigration decision to deny entry is an operational practice. You can appeal a decision made to be denied entry. Resulting in several days held by Thai Immigration in the Airport Immigration Detention Center waiting for a court hearing in front of a judge.
The 6 MONTHS Passport valid rule is universally accepted. Airlines can be fined for allowing you to Fly to Thailand. And are responsible to Fly you back to your originating city.
Simply book a one way flight ✈️ to Phnom Penh from Don Mueang airport within the number of days you are initially STAMPED in for...its cheap. Book with a site which allows for Free cancel within 24 hours. Print out the booking and cancel it.