if you already completed waiting game, and have not defeated the sorceress, you can weaken the barrier at any time with or without the rune, just need to do the 8 "Reduce casualties to a minimum..." quests again. Bear in mind that once you DO defeat the sorceress, you can no longer re-enter gloombright castle, regardless of being in a party with someone who does have the quest...
Not intentionally, but you can get ported in to the battle if you are in the gardens and somebody else has started the quest. This has happened to me once, and to someone else in our Butterfly raid another time. Have no idea what triggers it. In both cases our characters had previously completed that quest...so we got to do it again.