I make the slow cooker ones every year and they're great. Make sure to put a towel under the lid of the slow cooker so the condensation doesn't drip onto the potatoes and thin them out. Happy Thanksgiving!

I would do slow cooker if you have the space in your kitchen tomorrow.

I would go with whichever is going to be easier for you. Keep in mind that the make-aheads will need to be reheated, so you'll need to have a free burner or another means of managing that step.

I use a terry dish towel to absorb the moisture. Make your favorite mashed potato recipe and it will keep in the crockpot. If it thickens up a bit, add some milk or cream. I use cream cheese in my mashed potatoes, but I've never posted the recipe, sorry :)