Is it really HEAT STRESS?

6’ x 8’ room, 600w MH Vivosun in winged cool tube connected to 6” exhaust, 6” intake. AC window unit set to 78° on dehumidifier mode. Outside humidity is at 97% so intake is on low. 2x small fans blowing across tops of plants. Edges starting to curl up slightly, lights were 14” away moved to 18” last night.

Give it time. It will be fine. But don't have the fan blowing over them. Just have it moving the air in there and they should be alright. A fan blowing over them will cause them to dry and curl.
Good luck friend!