Not to put too fine a point on it...but Maeve was Immortal, and so was Aurora.
Sure, but that's change by an outside force of presumably greater power. The immutability of Immortals isn't absolute... but even non-mortals that aren't capital-I Immortals (such as regular fae) don't change
by their own choices and actions; it takes mortal free will to do that.
I think that's rather the point Mother Summer was making about whether Harry could resist his mantle.
The Knight Mantle doesn't make Harry an Immortal, though. It's a mantle specifically made for mortals.
I think the corruption caused by black magic is more like radiation sickness - get hit with too much and it changes you.
I think you have to
choose to use black magic, not just be exposed to it. I think it's just a result of the mortal ability to change one's own nature by free will, combined with the will-driven nature of magic making the effect much more direct & immediate.
(I tend to think that the 'mental ossification' of older wizards described in TC is actually the same effect resulting from non-Law-breaking magic.)
However, I propose that it wasn't feeding at all and most people have misinterpreted the scene. It seems more likely to me that the Blackstaff was attempting to make Ebeneezer more like it. It wants to be used, as often and violently as possible.
Certainly possible, but I am not sure the two ideas are really contradictory.
I think the Blackstaff is at least semi-living/semi-sentient and linked to its user's aura/life force/spirit/whatever, in a connection that goes both ways.
Not into black magic, but into the dark powers of winter - which I will say again are really not that different from black magic (just more of an icy theme).
I don't know. I think black magic corruption is more individual than that, for example using mind magic can make you want to fix people 'for their own good'.
Winter is much more violent and direct.
It might be parallel to First Law corruption (which is about killing), but I don't think necessarily corruption in general.