You are still able to progress the main quest even if Jake is dead (you will lack a journal update but thats it).
SPOILER:
After his death, you can pick up the amulet & choose to go kill braccus at the Church in Cyseal to the east (optional).
For the main quest, move north in Cyseal towards the fiery area & keep following the path, all the way up north there should be a bridge. This bridge leads to Luculla (which is where the journal update would lead you, if you spoke to Jake). From there onward you can pick up the main quest again.
Thanks so much! I think I do have the amulet you mentioned. Was hesitant to finish before, but did fight my way almost up to the church so am going to try this.
