I am creating a new faction for a vendor NPC, who will also be a potential follower as well as a key actor for my mod. However I've read that if I want to set relationships between that new faction and other factions, I must make sure that these other factions have exactly the same relationship set toward the new faction.

Does the later involves that I'd have to edit vanilla faction records (via CK's Faction form for each one of them)? How can I parametrize inter-faction relationships in the most "ethical" way, i.e. so as to enforce uttermost compatibility (without modifying vanilla records) ? Can a faction be the object of a Quest Alias?