The Ring stripped him due to inactivity , then along came Manny Pacquiao , all of a sudden Floyd and Mosley are fighting ranked at #2 and #3 in that division therefore no Ring Title at stake
When the #1 & #3 are Title holders then the Ring Title is on the line. When only one fighter is a Title holder only the there is no Ring Title on the line. The #1 & #2 can have a box-off if the Ring Title is Vacant, even if neither fighter holds a sanctioned belt title.
No that's not the rule, as Bradley and Alexander were both had belts when they fought in January. I'm pretty sure the board just makes a judgment call when the 1 and 3 fight and The Ring title is vacant.