I'm really not sure of what can or can't be done with NPCs and Factions; so if anyone can shed some light on the questions below, I'll be grateful!

1) Can you make an NPC join a Faction, just like the player does?

2) Can you set an NPC to a certain Rank in Faction X?

Not just modify their Rank by one step, but actually setting their Rank to N.

3) Can NPCs join more than a Faction?

In the CS you have only one "slot" to assign Faction, so that makes me think that the game can't keep track of more than a single Faction at the time for NPCs (also, the way filters for Factions work seems to confirm that the game assumes every NPC is a member of only one Faction).