I think it would be a good guess that most indies don't use paid beta testers, though I would also imagine that they test the game themselves quite a bit, and get others to test before releasing it to general testing.

Bete testers are rarely used enough. I recall Corey Cole saying that during his time at Sierra Quest for gGory 1 was one of the few games to get proper beta testing there because of what kind of a game it was. For most titles back then the testing wasn't that extensive either, but that beta testing really helped out to root out many issues form the game and that's a reason the first game in the series is the most bug free.

I don't think you can trade humble coupons. Got a Outlast DLC coupon but no option to give it to someone else.