You need to get your medium PH under control. Peat is super acidic. Also need to adjust water PH.
DE doesnt enter through an insects lungs. It scraps the exoskeleton, and dehydrates them like Roach Proof. ( Boric Acid )
Thrips can cause cupping by introducing tomato wilt virus. You need to eradicate them in the other tent, not just control them. You either have them, or you dont. They will introduce virus. They need to be made extinct.
Calcitic Lime will affect PH much faster than Dolomite. You need to use a ratio of 2/3 Calcitic Lime, to 1/3 Dolomitic lime. This also balances out Calcium, and Magnesuim. Calcitic for fast PH change, and Dolomite for long term PH stabilization. They work together. Especially in Peat based mediums.
Low Calcium, and Magnesium can also cause leaf cupping. Fucked up PH can cause nutrient lockout, and cause cupping. PH is beast at 6.2 in peat based medium. Also wouldnt have hurt to add Perlite, and Vermiculite to peat. Along with MYCORRHIZAE . Also best to soak peat before use, and pour off the excess water. Peat can be hydrophobic, and shed water. Dont get peat waterlogged, but make sure it wet, and holding some water.
Add 1 ounce of Calcitic Lime, and 1/2 ounce of Dolomitic Lime for every gallon of peat. For every 2 gallons of Peat, add half gallon perlite, and half gallon Vermiculite.
Humidity is also to high. It needs to be no more than 70% during veg, and 40%- 50% during flowering. 80% humidity during flowering is going to cause potentially serious mold, and/or mildew issues.
You say you have good airflow?? But are you getting FRESH AIR, and not just moving around room air???