So I'm trying to turn in the Lost Spider sidequest after dealing with the Shaman/Third Necromancer trial... but when I go to bring the spider to the spider rancher, he's got no model and the game softlocks immediately after.

Is there any possible solution to this (besides going back to before the Shaman bit)?

After turning in the cure ingredients and dealing with the shaman, the spider farmer should be dead (along with most of the imps in the village, particularly quest related ones).
You could just skip the quest; you'd only be missing a bit of experience and a poisonous dagger as the quest reward.