request for Harper, Rune & Adamantium hints!

I'm about to hand over Lanthorn to Elan and I'll like to complete all the possible quests first. So far there just isn't the Harper quest and can anyone furnish me info as to how to trigger the fight with the Rune? Lastly and most importantly, I'll like to know a specific plot detail at the end. you see, i managed to use adamantium stuff cos i loaded Imoen with it and kicked her to the copper coronet just before emerging to the surface. however i discovered that the adamantium corroded when i entered the area once again, prompting me to think that only that particular underdark exit area has adamantium check. therefore i'll like to know if the plot develops eventually in such a way that i could still collect the ada stuff from a stash later on without needing to pass thru that area. Thanks!