Would precisely be the point of corrupted Seanchan Prophecies, to assure that the Seanchan won't ally with the DR against the Shadow, but rather fight and try to capture him for the Crystal Throne. That's basically what Tuon wants to do, because of the Seanchan Prophecies, and what Semirhage advised Tuon to do in WH.

It also seemed that Ishy planned in TGH to have Egwene taken as damane to Seanchan in order to lure Rand there, so that he might be controlled by the Crystal Throne. We know that Ishy was Liandrin's and Suroth's master at that time, who plotted to capture Egwene.

I think it would be more difficult for Ishy to corrupt the Westlands Prophecies, because there were always AS, who preserved the true Prophecies. But as BS indicated, there also might be some errors in the Westlands Prophecies. Maybe Noal will shed more light on it, since Farstrider (who was also Ishy's tool) apparently translated the Prophecies, as we know from the beginning Prophecy of CoT.