Your plants are looking good! LST is the way to go, IMO. You'll get those lower/side branches popping out and growing up towards the light now.

I would really try to get a vent. system up and running. Even if it's just a small comp. fan exhausting out the top with a passive intake on the bottom, that is so much better than nothing! Do you know what the temps are in the cab? I think CFLs are underestimated when it comes to how much heat they can put off in a small space.

Got any close up pics of the burn? Could be pH issues, calcium def... anything really.