But why wouldn't Marcus mention it while monologuing to Kirk? It seems an obvious thing to say - "We have a fleet of these monsters waiting in the wings. Time to strike while we have the advantage." It would seem logical, as you point out, but there's no hint, which I guess is a failing of the writers.
I'd argue we already got the hint with the tabletop display...
But Marcus would want to lie to Kirk. His Plan A is to get Kirk killed with that reactor sabotage that allows the Klingons to catch him, but he's likely to have a Plan B, too, and the
Vengeance would be it. Ruining that surprise would be a bad idea. And Kirk would probably wonder why his exploration ship or flagship or whatever has to go do the assassination when there are proper war monsters available as well...
Then again, I also want to believe in an even deeper level of planning here, of Marcus contracting the penthouse massacre from Khan, so that a young lobster would be motivated to do the dirty deed while sensible veterans would be conveniently swept out of the way by Khan's superb marksmanship that would pose no risk to Marcus himself. I mean, if that attack (and the bombing of S31 that did no harm to the supply of supertorps) wasn't part of Marcus' plan, then there are too many coincidences there; if it's "cooperation" where Marcus' blackmailed-to-loyalty henchman Khan helps ignite the war as per Marcus' commands while secretly using this for his own escape plan, then the two villains would get much further in their plans before splitting and backstabbing. As far as the Klingon homeworld at least.
In ST:ID, it's two people who are superclever and thinking even more highly of their cleverness, working within very tight confines to achieve secret goals. In ST6:TUC, it's a much simpler scam where two people who would want nothing more than to backstab each other can still achieve their mutual goal of a destructive war by very simple means - by sowing chaos. The two plots are very different beasts, with Khan requiring lots of byzantine finesse to get himself and his crew out of Marcus' hands, but Cartwright or Chang needing to do very little besides shooting around.
Timo Saloniemi