I wandered aimlessly off to the ruins, found the ruins/angry spirit quest perchance, did the quest with the pig parts option, and looted the place. Just then did I start the quest with the river tentacle thing and went to the boat to examine the slime and got the antivenom recipe from Trish. I made the 2000% resist antivenom potion on the spot, and the quest still wanted me to talk to that elf guy in the tower or go to the elven ruins to find moss. WTF. 0/1 potions. What is going on? I made two of em-- nothing. BUG.

The quest updates when you loot the herb not when you make the potion with another ingredient (endrega fetus or something). You might as well do it for the xp.

Thanks. And another thing, a scotti'al elf enemy helped me kill one of those massive decloaking spider things (that appear after ~3 pods died). I'm pretty sure he got the killing blow on the spider--did I get the XP or miss out on anything?

note: elf wasn't under my sign control

I don't watch the combat log that closely :)
But I have the general feeling that kill xp is a pittance compared to quest xp.

I believe you are talking about the the three Cocoons and what appears after the third, is the Queen Endriag. I've only managed to draw one Queen out of hiding so far ( can't find any more at the moment,but know there are more as the quest is still active ). I don't have any help in that especially a Scoia'tael Elf. Still gotta find some bombs to destroy the Nekker nests. Those things are becoming a nuisance.
I forgot about the original question in my post ( got a bit off track ). I don't think it really mattters too much. I have accidently stumbled in to some quests, but got credit as well as the XP.