I'd stat Mort with Modular Abilities (or something very similar) instead of extra spell casting powers. GS spoiler:It seems to fit his spending an action to have a ghost 'possess' him and then gaining powers related to the ghost...even up to Harry's spell casting.
Yes, but that doesn't explain such things as Mort
gathering all those ghosts and freight training the body-snatcher.
That seems like a clearly high shift ectomancy spell.
As for Sponsored Magic, the books states "These power sources, called sponsors, are at least semi-aware, if not fully-aware, entities. Ancient and strange and potent, they have agendas of their own, and they view those to whom they grant a modicum of their own power as their agents throughout Creation." Mort seems to act out of compassion for the shades that wander Chicago, and not as their servant. Now, if you want to have a character with a nebulous sponsor in your game then that's your business. But as for the canon character? I'd say he's just a really specialized ectomancer.