For your gratin, try slicing the potatoes ahead of time and storing them in water....you've heard that before. You're right, it warps them. Try this?
As you slice, have wet paper towels ready at your station, make a 5x5 row of the slices, lay a wet paper towel on them, and then keep going until you have enough potatoes prepped out. To store them, stack them in a neatly fitting hotel pan and fill it with enough water to just cover them. Place a hotel pan on top, then weight with a #10 can. I prep out potatoes a day or two in advance for gratins and napoleons in this style. When it comes time to cook, you just have to pull them out. The top layer sometimes can oxidize a tiny bit, but it's a small price to pay, in my opinion, for having all the potatoes ready to use at a moments notice.
Seems like a lot of work to start, but if you keep doing something enough and taking the extra steps, you become faster at it. But I saw your prep list, that is a lot to do in 2.5 hours.