Do I need a return ticket to fly from Thailand to Cambodia for DTV application?

I’m not sure about anything else, but you don’t need to show your return ticket when you get to Phnom Penh airport.

To activate your DTV the first time you must enter thailand by plane - land boarder isn't working.

It depends. I have lived in Cambodia for 6 years with my Thai partner. Since we both have year-long Cambodian visas we usually never have been asked for return flights when flying to Phnom Penh. However, one time, in 2023, when my partner’s visa was expiring in 18ish days (we were returning to Phnom Penh to renew the Cambodian visa), Thai Airways made use buy my partner a ticket back to Thailand (since my Cambodian visa was good for another 6+ months, I didn’t need another ticket. ) Was surprised that Thai Airways did this, as it has usually been my experience that budget airlines do this.

Doubtful requirement on a regional flight.