As JB is a lazy writer, it makes sense for him to use the same possession methodology for Nemesis as he does for Denarians. We have seen Denarians operate in two ways.
1. Anduriel/Nicodemus work as a partnership. This parallels Nemesis/Maeve - Maeve was voluntarily working with Nemesis in exchange for freedom from her mother. Nemesis prefers this method.
2. Magog uses brute force to enslave the minds and bodies of his hosts. This parallels Nemesis/Cat Sith.
Now let's look at Harry and Lasciel. Through Lasciel's spirit, Lasciel attempted to form a cooperative (#1) relationship with Harry. However, as shown in Small Favor, Nicodemus believed that Lasciel would be able to use brute force (#2) to paralyze Harry long enough for Nicodemus to kill Harry.
My theory is that Mab is currently infected in the same manner that Harry was. When Mab touched took the Athame from Leah Nemesis was able to imprint on her. Nemesis is currently attempting to build a cooperative (#1) relationship. Mab is currently refusing and is too strong for Nemesis to simply take over like it did Cat Sith.
Why is this important? I think that Mab recognizes she can only prevail against Nemesis in the short term. Eventually, Nemesis will erode her defenses and start to take over. That is why Mab wants Harry as WK. We are vulnerable to ourselves. The Queens are Winter and the WK is part of Winter. Thus, while the Queens appear to control the WK, they are also secretly vulnerable to the WK. Mab needs a WK that will be strong and wise enough to exploit that vulnerability and kill her when she starts to lose control. One of Mab's goals in Cold Days was to start Harry on the road to discovering that the Queens are vulnerable to the WK. Harry now knows that the Immortal Queens can be stopped in certain places or certain times, and is on the path to learning that the Immortal Queens can also be stopped by certain people (i.e. their Knight).
This brings us to two possibilities. The first is that Harry will actually kill Mab, setting off the BAT. However, I think Harry is going to get around that and go with option #2. Harry will realize that it is Winter that is vulnerable to the Winter Knight, not the individual person in which Winter resides. Thus, Harry, will find a way to destroy the Winter mantle, setting Mab free of Winter. This will still set off the BAT, but it will create a touching scene in which Mab (and Molly) are restored to their humanity.