I know the xbox one outsold the ps4 in November.
Pro players are not the main audience for a game, they are a group to be considered, but for the few that "mastered" the game, a rather small group which holds no merits to the main audience.
343 is using dedicated servers since launch, as Twin explained dedicated servers alone do no solve problems, but there is no host advantage, why hosts then? Because you need a host as measuring point for server stability and client to client measuring, the other reason could be that the "old" games have not been build for dedicated server systems and they needed a workaround. For the rest the games are as similair as the originals
The reason Halo 5 runs better than the MCC, is that the game has been build for dedicated servers since development, trying to remodel games which are build for p2p networking, is a monumental task. Bringing apparently the problems which could not have been tested before hand.