Hm, so, one can write off the Vale as a lost cause. Likewise, one can forget about the Westerlands, even if Tywin is eliminated. Furthermore, the Westerlands are in control of most of the Riverlands and doing well for themselves in the North via proxy.
It is unlikely that the Stormlands will yield. The Reach's alliance is fragile at best. One can also probably forget about any support from Dorne.
...Hm, comme on dit, après moi, le déluge.
Perhaps the focus should simply be for Cersei to have a good time and enjoy herself as much as possible before the end.