You ever try tossing them in a little baking powder before the preheat? That was my wake-up call moment too, changed everything for me.

Honestly, grain direction is one of those things that gets blown way out of proportion. You picked a cheaper pack, the covers bowed, you pressed them flat, and now they're fine. So what's the actual problem? Plenty of binders use cross-grained board on purpose for certain projects because it bends easier along the spine when you're doing a tight back hinge. The real issue is that you didn't account for the humidity in your workspace. That's what makes board curl, not the grain. If you'd stored the finished books upright and clamped them for a day, you'd have saved yourself two weeks. Sometimes the "wrong" choice just needs a different handling method, not a full redesign of your process.

Wait, you had a printed list of codes in a filing cabinet and still forgot about it for six hours? That's wild. Props to you for being that organized on accident, I guess.