They could come true, and since we don't know their entirety, it is not as if they have any meaning for us. The one cited on the last page of tGS refers to Rand as blind, but it turns out to have been figurative, since he wasn't really blind in the event the prophecy mentions. Maybe they are true even if they are corrupted. As Moiraine notes regarding the writings in Fal Dara, prophecies from the Shadow come true too. The only reason why the corruption of the Seanchan prophecies would matter would be if we knew them and had to concern ourselves with whether or not they are reliable. If they are corrupted or are not, the Seanchan will still act on them as if they are not, which is all that matters where they are concerned. The only real issue of conern with this theoretical corruption is readers who construct theories based on other theories and use the idea of the supposed corruption of the Seanchan prophecies as evidence of Ishamael's scheming or whatnot.