Somehow with how the story is progressing I can see this series of events unfolding, because the mentor dying is cliche now and Jim loves abusing Harry.  Sorry if this isn't fully coherent, it is just my thought process in view...

1. Big event happens at "the wall" say book 18, with Molly and Harry at the spearhead. Refer to step 10 for the rest of this...
2. Meanwhile, Langley is accused with another good frame job of being with the outsiders or black council
3. Langley escapes, to fight the battle his way and leaves the council in chaos.
4. EB is then nominated and accepts the position of Merlin which means
5. EB has to give up the Blackstaff mantle
6. EB then gives the only Wizard he can trust that will accept the burden (LTW says absolutely no!) Harry
7. So now Harry has the Winter Mantle with Mother Winter's walking stick, which then really powers him up
8 But also has the urges that go along with Winter magnified, oh and the licence to do the wrong thing without punishment.

10. Harry is winter's general at the wall, and Molly is starting to turn more Wintery as well and being more of a frienemy to Harry, accomplishing Mab's goals at the expense of Harry.
11.  Harry works with the gatekeeper, and realizes even with his help they are doomed to lose this.
12. Harry has an ulterior motive while all of this is happening, trying to do his duty while getting in something under Mab's nose.
13. Molly "betrays" Harry, accomplishing Mab's goal and further complicating Harry's defense.
14. Harry fights back, after finding out Molly has been using subtle mind magic to make Harry more agreeable.
15. Then Harry with the blackstaff, turns the tide, but draws deeply on winter to do so, causing him to fracture even further towards the dark side.

So in the end of this series of events
EB is Merlin, and also leader of the grey council
Harry is the winter knight and the blackstaff
Molly is becoming more fully involved with winter, and loses Harry's trust

What if all "Uriel" did was transfer the mantel of his divinity to Michael.  Just like Harry holds the winter knight mantel, and Molly the winter lady, Uriel just gives the mantel of an arch angel to Michael.  I imagine the mantel acts in a different way from the Winter mantels however in that so long as the person wielding that mantel acts in accordance to "HIS" will that the mantel is returned to "HIM" even upon death. 

So once Uriel gave the mantel away he was able to act of his own free will because he was mortal again, but the only act of free will Uriel really has is to bestow the mantel to someone else.  I imagine that this hasn't happened often before but Michael was deemed worthy of this mantel and might actually be auditioning for a new part once he passes from the mortal coil.

Boiled down, this is what I think

Harry's Mantel=small fraction of winter's power (1-3%)
Uriel's Mantel=Large portion of Christianity's god power (10-20%)

Neither Harry nor Michael's bodies are truly healed by this, but their spirit grows to transcend the limits of mortality. My guess is the earring that Mab gives Harry is a temporary bit of her essence, that gives him more of her, too ignore the pain he is enduring.  Though I wonder since it broke on him if he isn't going to keep some of it without knowing it.