My point is, what is the point? There already is an alliance between the White Court and the Winter Court, no need for a marriage...
Marriage -- old-school, arranged marriage between nobles -- is almost always political. It's usually about taking an "alliance" and making it even closer, but sometimes about ending a war, or defusing a tense situation; also sometimes about getting rid of a problem-at-home by exporting them to the other court; or installing a loyal-to-you agent high within the other court; etc.
Mab works on just a single level of planning/intention about
never, so I'm certain there's more to it than that. But I'm certain that's
part of it!
... I think Mab is looking for someone who might be able to control Harry better than she can...
I'm pretty sure Mab isn't interested in
anybody who can "control (her Winter Knight) better than she can" (I think the idea would horrify her).
And I suspect Mab knows that Lara isn't the whamp for the job (Thomas is a *far* better lever upon Harry).
WE know Lara can't take Harry from how Harry basically let Lara "take her best shot" in the Raith Deeps, and not only did Harry not succumb... he maintained the focus and willpower to form magic -- on the fly -- that was both incredibly-powerful and pretty nuanced (with the hard outer sphere and layers of padding inside).
I bet Mab knows this, too.
... in my opinion it is just another sideshow we can do without.
I don't expect this wedding is coming to fruition. It is, as you say, a "sideshow," but it's a Harry-distracting, Harry-tormenting sideshow, and those are
very much in the Dresden Files tradition.
(It's also worth noting, here: Jim himself is coming off his
second divorce; this may influence marriage-related plotlines. (Similarly, I'm pretty sure we know WAY more about LKH's love-life & taste in men, courtesy of Anita & Merry, than many of us want to know (she certainly told us she was bi & poly before she "revealed" her bi/poly 3rd)))
...
I am starting to think, that this is about LARA!
Putting Harry in a place where HE can control Lara and not Mab or Lara controlling him.
This seems like a valid notion; or perhaps -- if not to overtly "control" Lara -- it's just about sending her (Starborn, Outsider-hating) WK into the Whamp Court to "investigate" her suspicions. Or just enact a "patented Harry-Dresden anarchygasm" and see what comes to light. But yes; I think it's about (or at least
more about) Lara & the Whamps, rather than about Harry.
I am getting the impression, this isn't about Harry or controlling Harry or putting Molly in her place or whatever.
Say rather (see above, "Mab works on just a single level of planning/intention about
never") this isn't
just about those things.
And I think she plans on this to just... "ablate" Molly's love, a bit.
She has the coldness of the Winter mantle acting on her...
She saw Harry getting intimate with Murphy...
Now Harry's engaged to Lara...
All of these things can have an impact on Molly's love. Mab I think sees any affection as a weakness, a vulnerability to exploit; and love the greatest of all. She hopes to wean the Winter Lady of such weakness. First, Harry; then the Carpenters.
I do not think she'll succeed, because she doesn't really understand these things anymore; in particular, she no longer remembers the strength that liking and loving can bring.
But I think her plans lay in that direction.
Harry, I think... she's throwing problems & challenges at him. A conflict is coming, and she is tempering him as a weapon. She wants him stronger, more flexible, more keen-edged. Also, I think she's laying other & deeper plans against him, stiffer challenges, etc...
this one she hopes will keep him more engaged, so he doesn't see the
next one coming.