But I don't think we can really afford to be leaving a sizeable portion of our forces at the Twins, which is basically ceding the Twins to the Lannisters. And that means losing any semblance of Frey loyalty. Moreover even if we did, besieging the Twins from the North means Tywin can cut us off from the North himself.
I'm trying,
really trying to figure out how you came to this conclusion. Like, what do you consider a weakly held castle? The only change I offered was that we not waste thousands of lives and keep the entire Northern foot out of the entire war like in canon, the garrisons at Moat Cailin (200 bowmen) and at the Twins (400 Freys, 400 Northemen under Tallhart) remain the same. Both are more than enough to hold Tywin back.
Tywin can't take Moat Cailin and the Twins. Period. This isn't a question of making an overly elaborate plan oversaturated with fail points to get Tywin to
not take the Twins easily, Tywin is not going to take the Twins.
He'll need to storm one castle and suffer heavy casualties, then his men would have to storm death alley across the bridge to try and strom the second castle and suffer massive casualties there, and at the end of the day all he has to show for it is that he "cut us off from the North"? Who gives a fuck? He just lost a major portion of his men, Jamie is fucked anyway because Tywin can't possibly keep pace with us, and now Tywin has an army severely weakend from the starting ~20,000 vs our army of 22,000+ Northmen and Freys and Mallisters plus close on the 11,000 Riverlords that Edmure gathers for the Battle of the Fords.
The Twins is not Harrnehal, it does not require an actual army to properly hold. That's the entire point of castles. Look at what Rolland Storm did in Dragonstone with like a couple dozen men - Over a thousand battle veteran knights and lordlings of the Lannisters were felled by the dregs that Stannis left behind. What do you think 800 soldiers in a perfect defensive position with good lines of fire is going to do to Tywin's host?
And in the time it would take Tywin to build the siege engine he needs to lay siege to the Twins, Robb can just go directly to King's Landing and end the war. So right off the bat there is no way Tywin would invest the time with a castle that gives him nothing.
In contrast, you want to
intentionally get Tywin between us and our infantry, with so many points of failure it isn't even funny. All it takes is for us to miss one of his scouts, or for one of our scouts keeping in contact with the other host to fall to the Lannisters, and we are fucked. Game over fucked.
And worse of all,
Tywin has every reason to figure this plan out because he starts already knowing that the siege is fake and the infantry can come from his rear.