The optimal temperature in the box is 27 degrees Celsius, but what is the maximum permissible temperature under the lamp near the tops of the plants?

in flower high temps can cause streching during first few week of flower, it looks like you are behind that, so only possible problem will be airy buds, not dense. if humidity is in such low rate, then all mold issues should be avoided too. 30 is really too high, but i grew in static caravan, so its tin can basically, had sometimes temps even higher, bud weight was only problem.