So, Hand's Detachment. It costs a hand and three feats, but then you do get a creepy crawly hand for a pet. Cool. But how does it work?
"Use the statistics for a crawling hand (Pathfinder RPG Bestiary 2 59) to represent the detached hand" - that's clear enough.
"save that the hand isn’t undead" - Okay, not Undead. Then what is it? I'm guessing it'll be a Construct, given that it doesn't have a Constitution score. But does it keep its Undead traits? Does it get the Construct traits? Both? Neither?
Also: do you still get the benefits (and penalty) of the prerequisite feats, once the hand is loose? It'd be illogical to get the boons of a possessed hand, when said hand is off gallivanting about the countryside. But the RAW is silent.
It's a magical beast, since it's a familiar. I think there was mention of treating its Con as 10 for saves/negative hp/etc., but I don't recall where or how official that was.
RAW isn't completely silent. You can't wield a weapon in the hand you no longer have, for instance. But I'd just apply "You cannot use the Possessed Hand feat or any feat with the Possessed Hand feat as a prerequisite until the hand is fully regenerated." to while the hand is gone too.
The trick is, unless you're playing in PFS, is to get the Countenanced Carbuncle feat from the Vampire Hunter D book. It does the exact same thing as Possessed Hand. So while they won't stack if you have both hands attached, you'd still keep the bonuses if you have one playing Thing.