As for 3, personally I would kill the mouse and throw it out, then hope that the others do not die as well. Alternatively EVERY time you go get mice you could sign a live animal waver and return the animal if it ends up ill within the first 24 (or 48?) hours. But I would not feed my snakes sick mice, and if any of the mice got sick I would probably scrap the entire lot of them. I doubt the snake can catch anything off the mice, but you could (perhaps Toxoplasmosis) or other animals you have could (same as before) or your snake's leavings could contain it. But yeah I'm paranoid and er on the side of caution.
Again I have never had a mouse die of illness or natural causes after getting them from the pet store. Well unless you want to consider a snake eating it a natural cause. In fact I'd probably call the pet store complaining and not go there anymore. Kinda how while this girl I know says every pet store has dead fish I say that if you see a dead fish that means that the petstore is not doing it's job (aka they should have removed the fish and probably quarantined the rest) and as such should not be purchased from.