Maybe I was overthinking that bit about the MP. Where I am now I'm only at 23 points or so and am sort of hoarding 'em.
Regarding the +s, for some reason I thought there was a way to increase it for the Phantom/Soul Weapons, but I guess that's not the case by the looks of it. Oh well...
As for Erebus that does sound rather good, so long as the other team members cover the essentials already. The Kamihime wiki doesn't seem to have %s on the Vigor buff unfortunately.