Ha, I am the last person who should give advice here because my first attempt at a screen was a fitted sheet still on the bed, and I sat there wondering why the movie had a weird lump in the middle during every dark scene. My wife still brings that up at dinner parties. What you found makes sense though, the gray kills the light coming back at the screen from lamps and walls, so the dark parts stop looking like fog. I run a dark gray curtain panel now at about 110 inches and it holds up fine, the only thing is you want something with a little weight so it hangs flat instead of curling at the edges. Steam is the real hero of this whole setup, wrinkles show up way more than people expect once the picture gets big.

Steaming up your shop like a wet basement is annoying, sure, but is it really that big a deal? You dropped $260 at a supply show, which is basically the place where people buy stuff they don't need because the booth had free coffee. A hot towel cabinet is meant to hold already-hot towels, not heat your whole room. If it's fogging your windows, just crack a door or run a fan for ten minutes. Worst case you got a fancy towel warmer and a free sauna for your shop.