I actually think the death cult from Mythshore doesn't have anything to do with what's happening with the orcs , while both have the eldritch horror signature mark(I say that because of the orc brute sanity drain move) they are way to different from one another to be related, since generally speaking this kinds of things have a modus operandi, like the nameless likes to fuck with the mind(everybody that's cleansed from the curse of lust says that their body was kind of in autopilot and they had no control over it) and the witch likes subtle behavioural manipulations (like the aphrodisiac around her hut) but the orcs are having their natural disposition amplified and warped and Mythshore was completely run over by the "we tried to summon an elder god" vibes. In short I don't think that it's possible for the both to have the same third party involved. Also the teacher could be made to help against his will in some way making him unable to keep contact or alternatively he could be farther north closer to Durig's home(though I like the traitorous teacher idea more, but that's because I like drama and trauma in stories even when it's at my characters expense there's just so much to explore and develop that way¯\_(ツ)_/¯).
Delivery people could simply ask around Bareshade where is the herbalist's hut since everybody there knows where we live and I'm pretty sure the message was dropped at Bernie's and then he went to the hut and left it there since it was him that handled the shipping and he also got a letter. And the reason people cannot reach Mythshore is the orc's blockade not the avalanche that keeps people from going north(I know it was Xerxes-gen who mentioned the avalanche but I'm kind lazy right now and don't wanna write another comment Lol).