Regarding the horcruxes, I think that with one book left we'll probably safely assume that the horcruxes suspected by Dumbledore, including Nagini, will be correct. Just too much distance to cover in too short a time ... They have to track down four horcruxes, destroy at least three (if the locket wasn't already destroyed. Note that Marvolo's ring still existed, even if it's "horcruxness" was destroyed) and wage an all-out battle with Malfoy, Snape and Voldemort, probably in that order. Add to that some time on Privet Drive and a wedding for the ages, and I really don't think they will have time to be at Hogwarts. Maybe they'll file for their N.E.W.T.s via Kwikspell. "Defeated the Dark Lord Six Times" will look very good on an application.