Read in a different thread that you can't assign a named settler to a Supply Line. (Can't remember her name right off, but the gripey/crabby one.)
However, I tried to assign a different, named one to a Guard Post and that didn't work either.
Can you assign 'named settlers' to anything specific?
Best Answer
From personal experience, I can tell you that it is possible to assign at least some of the named settlers to at least some duties.
For example, here's a few assignments I have done before. A couple of these are still in place on my current game:
Personally, I haven't encountered (or just cannot recall) a case where I've been unable to assign any settler to a duty of my choosing. But, I haven't really experimented much with it either.
It is conceivable that there may be some rules in place, affecting certain critical NPCs, in order to protect the story flow. But it would take a good deal of thorough playtesting to enumerate all those rules (determining which named NPCs can or cannot do which jobs).
The first case you give seems to be one of the most likely ones to get blocked in general though. You're trying to assigned a named NPC as a Provisioner (that's what people running the Supply Lines are called - more on that shortly). That's something you generally wouldn't want to do to a named NPC - both in gaming terms and in programming terms - for two reasons:
At this point though, I'm just speculating. I have not yet tested assigning a named NPC to a Supply Line. If I get the chance, I'll update this answer. (Or someone else who already has is welcome to.)
Tip: If you can, you probably want to avoid assigning named NPCs (especially companions) to stores unless they're "Tier 4" vendors. Otherwise, their normal dialog options tend to get in the way of trading a bit. Even some of the "Tier 4" vendors have this problem (The one mentioned in spoiler text above still makes me wait through his rather long ambient ramblings before I can actually Barter with him.) but at least they make it worthwhile.