Ok, let's try to sort this out.
1) The quest Spicing Things Up has no influence on your reputation with the Loreseekers, irrespective of the quest path you choose. The closed grates inside the Great Library are supposed to open once Forgotten Lore is completed, but you may need to rest at an inn before this takes effect. The reputation of +15 you mentioned is only necessary if you want to learn skills from Gebadi. There's no reputation check for Secrets of the Dead or The Lost Coven whatsoever.
2) There are certain areas where enemies respawn at a constant rate. The Ghosts and Wraiths in Deadwood are one example. You will only get the full XP when killing them the first time after entering the map, while each repawned enemy will only yield a minimal amount of XP. This is intentional to prevent too easy XP farming.
However, the Corrupted Sabercats in Fögas Forest do not belong to this kind of enemy. I'm not sure what's going on there.