Just make sure you give the Archers some Flak ammo so they can take a few dirty clanners with them... 
You can do that with regular HE, it gets Flak tags. That and...well, a
lot of LRMs being thrown around in that last battle, the Falcons won't get away entirely unscathed.
EDIT: As far as the history of Zoetermeer goes and the Wieczorek family, they managed to escape aboard the family's modified Triumph DropShip, along with some of the unit that was providing an honor guard - some of the infantry, I suppose, maybe a liason force with the family's own house guards. That puts the granduncle aboard the DropShip, and makes him able to tell the stories of fighting the Mariks and Kuritans and finally the Falcons, as well as telling her how awesome the Tamar Pact was, to little Mirage.
As far as the Falcons go, it's not stated in Sarna but they were searching for a Star League facility on the planet. They were unsuccessful; in this canon it was an abandoned facility that didn't have much except starmaps and some old records from pre-Amaris times. That's what led Mirage to Erdvynn in the first place, and whatever small outpost there was was empty otherwise. So there'd have to be some kind of Falcon garrison on Zoetermeer, with some Warriors to go search around for the facility and come up empty-handed. The Emerald Talon Naval Star withdrew after the 10th Donegal and 12th Star Guard forces surrendered, but there's no mention of a PGC being moved in place...but there were defenders to fight the Wolves for the planet in 3070.
New idea: they never surrendered to the Emerald Talon Naval Star, but instead of being brought down by a swarm of aerospace fighters the ETNS jumped out and ground forces were brought in immediately afterward. Mirage decided to stand and fight, taking on the second-line forces in guerilla style battles before eventually being run to ground and destroyed by the Falcon garrison troops.
This would explain some of how the rest of the family escapes; they waited until the WarShips left before making a break to a waiting JumpShip at a pirate point. Mirage would have sent her tech and admin troops with them, after loading up as much ammunition as she could handle with the infantry driving the trucks; it's one of these techs that ends up being little Mirage's favorite granduncle. He'd have intimate knowledge of just how to shove a Thumper into an Archer and make it fit if he was one of those Techs that fled the world with the family dependents.
I'm liking this idea better, honestly, partly for granduncle being one of the 'Mech Techs and because it means I can someday game out the bitter end of Wieczorek's Archers...with a big emphasis on 'someday.'
As far as Kampfgruppe W, well, I suppose I should work up a company of 21st Centauri Lancers from the RATs in FM3145, and add in offboard fire support with the howitzers. I just wonder how much salvage that'll net me, since I have a full salvage subcontract with the 21st. Considering it's Totally Destroyed if the CT is cored with an AE weapon, and unsalvageable, I'm gonna have to be careful with my fire...and rely on my tanks for getting me shiny goodies.