...in my load order, yet only ONE shows up: "MercenaryPack.esm", which is the one (of 4) I originally purchased prior to FNV's release. Since then, I've purchased through Steam the other three Official POP (Pre-Order Packs), yet I still only show/see the originally purchased "Mercenary Pack" in my plug-ins list/load order (Mod Organizer).

Why don't the other three (ie: CaravanPack.esm, TribalPack.esm & ClassicPack.esm) show up in my plug-ins list/load order? Despite them not being present in said plug-ins list/load order, they DO effectively show-up in-game just fine...

Sounds like an issue with Mod Organizer. I have no idea what the issue is since I have never used that program.