Do they ever question the return flight date when extending a visa in Thailand?

There is no requirement to show flight details for an extension. There could be if you’re trying to get the “extension denied” stamp for 7 days.

A flight out of Thailand is not normally a requirement for an extension.

The only time you'll be asked to show this when you arrive is if they're looking for a reason to deny entry.

It's usually your airline that will ask you to show this, and they may not let you board your flight without it, but only if you're traveling as a tourist. Someone with a non-immigrant visa has no requirement to show proof of onward travel.