I don't see how locking the screen would help, and after reading your post, reassigning the power key wouldnt do any good either. I understood it as your phone going into suspend instead of just the screen turning off. Anyway, it definitely seems like a hardware issue because hundreds or even thousands of people have used the roms you've listed without widely reported screen power issues.. The recent change in symptoms may have easily been caused by a gentle shock or firm press on the keyboard (I'm still thinking it's a loose ribbon cable).
Maybe it's just me, but I hate going for problems, waiting for 2 hours, only to be told that it's all normal and they were unable to duplicate the issue. If you're at all tech savvy with a couple of small screwdrivers, you'll be able to fix it yourself and avoid inheriting somebody else's problems when they stick you with a refurb
Otherwise, it's worth flashing back to stock and see if it continues now + find a way to make it happen on demand for Sprint's benefit.