Addendum to the troop dispositions again, but even without the Riverrun and Raventree sieges the problems of bandits, rebels and outlaws also prevailed in the Riverlands to the point where Randyl Tarlly had his entire army at Saltpan/the southeast Riverlands at one point, I just find it hard to believe the Lannisters could afford in turn to leave such a small occupation force for the rest of the Riverlands until the area had stabilized a bit more.

I think the issue is that the Lannisters are mostly tapped out. They started the war with 45,000 men under arms – they lost 3,000 men at the Whispering Woods, 8,000 at the Camps, and 10,000 at Oxcross, and an unknown number at the Green Fork, the retreat from the Green Fork, raids and counter-raids in the Riverlands, the Battle of the Fords, and the Battle of Blackwater, which I think adds up to anywhere between 5,000 and 8,000 men. 

So at the point that Tywin dies, I think the Lannisters have about 12-15,000 men in Tywin’s army and another 2,000 or so that Daven has pulled from the garrisons of Casterly Rock and the Golden Tooth and the survivors of Oxcross. Of Tywin’s army, 2,000 gets sent to Dragonstone (1,000 of them die), 800 go with Jaime, 450 or so go with Tywin’s funeral escort, and the rest are demobilized when their lords leave for the Westerlands.

It’s not a question of affording any more – those 10,000 or so men have been under arms for the better part of two years, much longer than any normal term of feudal service. Tywin’s force of personality kept them them in the field, but once Tywin is dead, they’re not going to stay put, especially when the identity of the actual lord of Casterly Rock is so ambiguous.