I've seen a bunch of guides saying I should get an option to add the jarrin root to the recipe but yet I never do. I just get options to add random things like a giant's toe and gold coins. In fact, I don't even see jarrin root in my inventory. Was it supposed to have been given to me automatically upon accepting the quest, or am I supposed to go out and buy/find it somewhere? If the latter, where? Thanks.

Mmmh Astrid gave you one.... You didn't taste it ...? Did you ? (unlike what she claims it doesn't kill you) If you did you probably have to find another way to kill the "emperor"

No idea, probably user error but I restarted (luckily I had a save before I started the quest and didn't play too far). This time I got it.