How long should I cook sliced russert potatoes for mashing?
I think you are overthinking this somewhat on the starch front, I don't think that soaking and rinsing overnight, or the mandolin cut will reduce glueyness. It's the choice of potato and how you mash it that makes it gluey or not. If you use a ricer and non-waxy potatoes you should be fine. Just add lots of butter and they'll love it! 

Slicing the potatoes thinly will make them cook faster due to the higher surface area, it also removes any room for error - the difference between undercooked and overcooked is all of a minute. Thin slicing can also be a problem for water circulation, leading to the potatoes on the bottom being mush and the top ones being raw, putting them in the pot vertically will prevent this. You are using 1/2 inch slices so you should be fine, but if you go thinner be aware of it. If you are doing thicker slices on a mandolin keep in mind the curvy end pieces will cook faster than the middle cuts. As for how much of a cooking time you'll save I think it may be a couple of minutes over cutting them into quarters, but it's hard to say exactly.