1) If you kill the bandits in Kurhu (which is something that players roleplaying good aligned characters might feasibly do) before you get quest 7 to murder Adric, the quest is skipped. This makes sense, however after you complete quest 8, Adric never appears at the guild and Ethalvora disappears from the guild like she would if you would choose Adric's route. This makes the quest 8a to fix the Weir Gate impossible to complete, because you're missing both of the NPCs that would give you the lens back.

Logically, if the bandits are dead and they don't kidnap Adric, it should mean that he arrives at the guild without any complications and not that he disappears from the game completely.

2) If you choose Adric's route, but visit the Weir Gate before you speak to him after completing quest 8, he won't give you the lens, because the dialogue lines are filtered to lower stages of the quest. This again makes quest 8a impossible to complete.