So does the very short N make it hoppable?
No because he still pays the
last N which is the key to making it unhoppable. I'm trying to envision if there might be some way to strategically mine going back and forth between two pools where the N ends up being wildly different durations but I can't think of one. OOC might be able to chime in here.
General PPLNS is provably fair, regardless of N. Even if N was just the last share, it would be still fair (and equivalent to solomining). I think Slush's version should be fair, but I can't be certain without a proof.