Mercury Records were an arm of Polygram records who Bon Jovi were signed to.
Universal Music bought Polygram and its subsidiaries a few years ago.
Bon Jovi are now on with Island/Def Jam Records which is under the (rather large) wing of Universal.
In short, the major record companies have all these minor arms below them.
As far as Doc MCGhee goes, he was their manager in the BIG years, got busted for some major dope trafficking etc etc.
JBJ and the boys, after almost falling apart in the late 1990s, decided a clean slate was needed if the band was to have a future, Doc got the flick and BJM (Bon Jovi Management) was formed.

Sorry if this is garbled, but it's 2am here in OZ and I'm damn sleepy