Interesting problem,
DJ Short Suggests the usage of HPS:MH in a ratio of 1:1.
I've used HPS:MH in that way, but never had problems, it does yield differently.
It looks like the pistils are dying off early, and is that why you guessed ethylene problems?
Hmm, A few good suggestions out there, you're not running a CO2 burner, so we can mark off atmosphere contamination in that way, And somebody suggested using a tomato plant as a ethylene indicator. And a few mentions of offgassing.
I would pull a few plants and put them in a totally different room, just to see if it is the room, until you can prove that there is no ethylene in the room, air testing, or ethylene detector kits. See if you see a different growth from those isolated.
Gotta ask you're not using any agents like "Switch" or "reverse" ..?