Lamp Hight

Based on the temp I would say u need something to cool the air down... I had the same problem (not as extrem as in your tent) If u have the possibility to move your setup to a room which is colder I definitely would recommend it otherwise get one more fan, it could help a lot.

Normally, during Veg, you would want the light closer I would think, as those leaves store the energy the plant will use to create buds, and the more they can get the better. I too am using a 600W light, and I keep mine at 18 inches right now, but I also supplement with some piddly-CO2 from fermentation (every bit helps). I would think if your 5 variables are tuned in correctly (light, CO2, water, nutrients, air movement), you would be able to get that light closer if anything, with some positive gains. Check out Ed Rosenthal's book on growing Cannabis, fantastic resource that explains this way better than I ever could.

Everything looks good. I would keep it where it is.