I sold some Tenebrium weapons to Bjorn, the smithy in Silverglen, and now he's half dead with the Rot (damn, this game doesn't hold any punches). I tried using a Bloodstone next to him, but it didn't cure him. Does that mean I'm going to loose a valuable vendor, or is there a way to cure NPCs? There are no relevant dialog options: he still thinks life is grand because I rescued him, his wife and their beast of burden in an earlier quest.

Well it seems Rot doesn't kill the vendor, but still, this means that you cannot sell Tenebrium equipment to vendors without infecting them. There's an element of realism there, I guess, but a few appropriate, accessible and immune vendors would be nice.

There are some
- Skelet before Hunters Edge (you infect him but he take's no Damage)
- merchants in Secret Stone
- travling merchant in End of Time
- Elementals in End of Time

Yeah, but none are really appropriate. I saw the skeleton (a junk collector, not an 'appropriate' merchant), the entire town of Sacred Stone (not Secret Stone) went hostile on me and no longer exist, and I dislike going back to EOT for trading... seems cheesy.

Immaculates in Hiberheim ?
Also it is realistic since only the Immaculates can handle Tenebrium without getting Rot.

There is nobody left alive in Hiberheim. I don't go around indiscriminately wiping out populations, but apart from a very few exceptions (gate keepers, etc.), the Immaculates have attacked me on sight, even though I have the amulets. It's why Sacred Stone is also sans population. And I said appropriate, not realistic. My initial sale to Bjorn was done without thinking re. Tenebrium, and I accept that. What I don't understand is why I could not heal him with a Bloodstone. That's a silly oversight by the devs, if you ask me, after all, just outside of town you heal Bjorn's beast of burden with one.

My mistake with realism/appropriate.
Strange with the attacking on sight thing never happened to me, maybe a bug?

No, I don't think it's a bug. My feeling is that there are so many ways to go about tasks and quests that one person's experience will likely be very different from another person's. With the lack of 'hand holding' and scarcity of obvious clues on how to follow (let alone complete) quests, there's bound to be situations like this. I would be very surprised (and a little disappointed) if it was a bug.