Do I need an exit ticket when entering Thailand on a DTV visa?

No

The other alternative is to use the site onwardticket.com and you purchase a reservation code. This is worked for me 100% in many countries. It actually is a legitimate reservation but you don’t have to exercise it so in other words you’ll get a ticket reservation that shows up like a normal ticket you show the people at the airlines and then you’re all good. I just use this service a couple days ago called it only cost 12 bucks. The other one I think is 18 and this one worked flawlessly also.

Fly with Thai airways and you won't have any problems. They know about DTV

get a separate one way back full refundable ticket and as soon you cross the immigration refund it, why take headache?......

Not an issue. Airline may ask how long you are planning to stay but once you show DTV at the counter it’s sorted. Airport Immigration doesn’t ask return ticket or any questions once you show DTV.

I entered on 11 April without return ticket.

The airline may have an issue if they do not understand the DTV. Show your DTV at check in and you should be fine when they realize it is a long-term visa. I have always entered on one-way tickets with the DTV with no probs and many others the same.