And that is mentioned at the end of Peace Talks when Vadderrung and Company try to devise a defense strategy for Chicago.  That the Fomor reign supreme under water and none of the powers at the Peace Talks has much knowledge and no influence there.  So in that sense it is easy to see why they would be totally hoodwinked by the Fomor.

That was Cold Days, and it was Mother Summer who took Harry to the Outer Gates.  The only experience he had before seeing any of Winter's army was in Summer Knight.  One would think that while most of Winter's forces would be at the Gates, there would still be at least a couple of brigades in and around the Court in case Mab needs them.  And in Battle Ground both Mab and Molly were in Chicago fighting, can't remember if they brought troops with them or not, but even so both the Winter Queen and the Lady were in Chicago while supposedly the Gates were under siege with only Rashid to lead the Winter Army, unless there were some other of Winter's generals.

The problem is, at some point star born Harry is going to have to unite all these factions otherwise they won't have a chance against Nemesis and the Outsiders.

Just a thought, Mab's intentions for the marriage might be a stronger alliance between Winter and the White Court.  However as many of you pointed out, that alliance might make uniting everyone against Nemesis/Outsiders in the coming BAT a whole lot harder because of infighting between the allied factions.. Hmmm... I wonder if this will be an instance where Mab is totally wrong in her planning?  If she is, will she openly admit it?

The Hospitallers still exist.

There are about 40 "First Class" members, 600 "Second Class" members (mainly titled "Knights in Obedience") and 12,400 "Third Class" members.
(per Wikipedia, fwiw)

This is a very-interesting notion.

I don't think Jim's going to write it this way; but it's very interesting!

The pieces hang together well-enough for me to now think it's likely that "replace Lara's Hunger-Demon with a Wintermantle" may very-well be part of Mab's planning (if only 2nd or 3rd tier contingency).

I've intentionally not specified the Queen-Mantle... Lara of course hardly qualifies as "virginal" but maybe the magic can admit of a "from now on" definition.  It'd be incredibly-ironic for "Queen of the Sex Vampires" to become mythically-virginal!      And we know (from the disastrous Carlos/Molly event) that Jim's very willing to torment other characters than Harry, with this sort of thing.  And it'd give Molly an "out" from the Ladymantle, if Harry could find a way to initiate the transfer without killing the current owner...

Or, just do the "normal" Queen dies + Lady-becomes-Queen + Ladymantle-needs-host sequence, with the Winterknight's fiance close to hand.  Ratchet the Molly/Harry tension another notch, I'm sure Jim would like it for that if nothing else.

Or just some direct Lara-gets-Queenmantle path.  That might eliminate the "marriage" elements, though, if there's any incompatibilities between the mythic Winterlaw ideal of "marriage" and the Queen/Knight relationship.  I think Jim's more-likely to write the complex one that loops-in Molly, ratchets-up tension, and tweaks Lara's twerk.

So, lots of elements come together very nicely indeed!  I like your theory a lot, @Avernite!