With the NPC update where you can zone your NPC, you can now dc and your NPC will come back.
As for the Prom quests, I have been able to simply zone out, and zone back in (immediately) and restart the quest.
This has been handy when I want to mess around in the proms, since I can go in, kill whatever, go to the top floor, and just let my npc go (by zoning into the tower), and on the next prom just bring her back out.
I plan on using this to get my slivers as well... just haven't had the spare reraise hairpin to try and get there (assuming I will probably die on the fourth floor... Though getting there is harder then running around on it)