Hmm, don't know if this is actually useful, but a mix of DirtyBlock and update some of the stats on the stats on the fly would probably be the best alternative, the slave doesn't sound very good.. The user stats for an anime/ep/file could be probably be calculated on event (add file to mylist, remove file from mylist), file/ep count the same.
group stats could use the dirty block, no loss there.
user stats is the main problem, definitely dirty block here, but i would make the added to db stuff on event (add/remove anime, last anime added to mylist, date of last mylist addition) the rest i would make on request of info, the user probably won't request the data much so no much need to update that data that often.
Though the trigger issues brought up is tricky indeed... -fahrenheit