The main quest doesn't save progress mid instance, if you disconnect, log out or leave mid quest, you have to do the whole thing all over again. Theres nothing you can do to retrieve it, you just have to do it again.
As for the tharn, most bosses have mechanics that don't encourage being far away. It's probably his mechanic, just check the kill feed if you die and see what it is. If it doesn't look right, report it.