Establishing a secret group of Darkfriends in the White Tower doesn't require precognition at all - how could setting a group of people loyal to you to infiltrate what could be your greatest threat not pay dividends? The worst that could happen would be that the White Tower uncovers the Darkfriends and removes them, but then Ishamael hasn't really lost anything, has he? Besides, he had no far reaching plans that required the existence of the Black Ajah, he simply used them as an available resource in his future plans, when he knew them to be available.

The Seanchan are more in line with actual convoluted, far reaching plans, because Ishamael actually needed to return at the appropriate time. But then again, it requires no precognition either if we accept that Ishamael could simply have gone to Seanchan and influence the Empress and High Blood to commence with the Return. And obviously I don't think that Ishamael could predict that the Seanchan would return just as he wanted them.