The acolyte you're missing is actually the temple one go stalk the suspicious warriors and they'll lead you to the one missing(the one suffering from the curse of lust has nothing to do with the missing acolytes).
To help the shopkeeper you need to craft heat occissia and then use the heat occissia to craft spectral syrup.
You have a random chance of finding an oak tree by roaming the woods close to your hut.