Can't cancel the Threnal quest line midway?

[Maleth]

I just recently decided to run Threnal and while I was half way through, a friend wanted to join. Imagine my dismay when I couldn't cancel and restart from the beginning. I thought maybe it was because I had to update the quest by talking to an NPC. Did that and still no go. Am I missing something? I've always been able to cancel before without a problem. Now, considering how crappy the XP is for the last step of the library, I have no interest what-so-ever to go back in 2 more time to get elite favor and will likely want to just go to the last step and cancel, but they won't let me. What gives? Is it a bug or are they just being hard asses for the Threnal quest lines of all things?

[Gorby]

Threnal East 1 is an easy run to the end. Enter, haste, fire resist, run to the end and whack the Fire Reaver a few times for a few K XP. Recall, reset the quest chain and repeat.

Turbine decided they didn't want people doing that anymore so they changed the quest line to be non-abandonable.

[QuantumFX]

I guess the devs were thinking that since parts 1 and 2 were so much fun that forcing you to do part 3 in order to repeat it would be “worth it” from a player’s PoV.

Really, part 3 needs to be revisited. Either remake it (ex. the Korthos Island quests), shorten it (ex. Desert Caravan), make Coyle more like our good friend from Ritual Sacrifice (You know: Fairly ineffective with a bunch of Hit Points.) or allow us the proper amount of revenge on him (Let us knock him out (Bluff/Stunning Blow DC/Stunning Fist DC), cast water breathing on him, stick him in a dunk tank so the mobs can't get at him and allow him to drown after the quest has been completed.)