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