Not everything here adds up. I went back and reread everything. You said 400cfm through carbon filter.
A 160cfm fan plus a 240cfm fan does not equal 400cfm. Not possible
The total CFM through a fan cannot exceed blade design and fan speed. Booster fan just means the fan does not work as hard. A 160 CFM fan through a carbon filter is probably no more than 80 CFM at the outlet you have a 65CF space with 12 cfl. 20 degree temp rise with low air flow is not good but it will not kill plants.
If your carbon filter was using the 240 CFM fan without the 160 it would actually move more air. The fan you have blowing through the intake will decrease resistence to air flow and reduce the load on the exhaust but there is no way to increase the CFM of the exhaust fan. If you stack 10 240 CFM fans together you still only get 240 CFM unless you increase fan speed and velocity.
Pics are not always as they appear. I cannot see the bottom of the pots well enough to tell what they are. Do the pots have holes at the bottom exiting the sides of the pot. You got to have air available to the roots. It looks like pots on carpeted floor with no saucers. If the pots have bottom holes that are plugged by carpet pushing plastic into them that is the same as no air hole.
97 degrees is not good but plants plants survive higher temps just fine. Given the size of the space the 88 degrees you had is not bad.
If you suck 240 CFM through a 65 CF air space yes it should be hard to keep mylar on the walls. The intake has to be able to handle more air than you exhaust or it creates a vaccum. Air flow across the space is irrelevant unless it overcomes intake or dries plants out too fast. Your plants do not know or care which way the wind blows. They just need fresh air and a slight breeze.
