To answer the actual question, you could indeed put Cure Critical Wounds into a runestaff. You wouldn't be able to use it unless you had Cure Critical Wounds on your spell list, though, and since standard runestaves only work with arcane spellcasters, only the bard is going to get any use out of that.

As for the second question, the example given (a hexblade using a 2nd-level spell slot to cast Suggestion, which is a 2nd-level spell for them but a 3rd-level spell for wizards) would seem to indicate that you expend a spell slot equivalent to what you'd use if you, personally, were casting the spell from your own slots. I guess you could have the staff crafted by a duskblade or something to try to reduce the cost of putting in Polar Ray, but that's it.