Yeah probably has a lot to do with PG/VG mix and I'll wager some flavors produce more gunk than others.
Absolutely
It is not impossible to run up to 30ml liquid through a carto. It has been done. Considering the life is intended for one fill up, should you get 15ml out of them then they are performers.
Wrt washing them I simply don't, it is not worth the effort for the results IMO but YMMV. When you do get that burnt taste, even though it is ever so faint, toss 'em. The damage is done and they will not come back. I have conducted enough autopsies to know