1. We have seen that mortals can make deals with beings from the NeverNever for power. At times this involves a connection between souls like that which makes a white court vampire. The human gains power from the NeverNever creature and that creature gains an ability to effect its host and the mortal world (i.e. feed in the Hunger's case).
Well... deals for power don't generally seem to create that kind of close connection. Sharing a body with some kind of Nevernever entity seems to either be "natural" (I don't think there's any evidence that White Court vampires or lycanthropes were created by mortal spellcasters) or the result of something really, really powerful (Denarian Coins, Faerie Knights' Mantles).
The hexenwolf belts might be a counter-example, but they're really not that powerful in the grand scheme of things, and IIRC Jim has hinted that the theory that they came from the Erlking is correct, in which case that's another case of something really, really powerful setting up the situation.
Maaaaybe the Archive was something like this, but if so that's probably a case of "way, way beyond what modern wizards can do" like Merlin's creation of Demonreach. Bob is a Seriously Big Deal to the White Council and he's not nearly on the Archive's level.
If setting up this sort of thing was within the power of a standard wizard, I think a ton of crazy warlocks would have awesome physical powers to add to their spellcasting ability.
What happens when a human soul makes a symbiotic relationship with an angel? I think this is very, very rare, since obviously angels would be very nervous about bequeathing their power to a mortal who can misuse it.
I think that the "normal" way of bequeathing angel power to a mortal is granting Soulfire. Harry presumably isn't the only mortal spellcaster to have gotten this, since
the skinwalker in Turn Coat comments that it's strange that he can use it without faith. Perhaps it's something that a lot of Saints have gotten in the past.
And on a much higher level, there's Uriel lending his Grace to Michael in Skin Game...