[Bill]

'Could a freediver in trunks really lose enough buoyancy to keep them from regaining the surface?
What depth would that be likely to become an issue?'

Easy to test. Pick an easy depth and have a safety. Dive to the bottom and exhale all your air. Your buoyancy will be the same at any bottom less than 60+ feet. If you sink another 60 feet after you exhale, you could lose a few pounds more (and do a lot of damage). Fresh water can be 5-6 pounds less buoyant.
I've seen a few of these water tests and it's hard to find anyone over -10 pounds (You need very low body fat) and 10# isn't too hard to get moving up.