I don't think their fates were switched. I'm fairly convinced RJ left no details about Aran'gar beside the fact Graendal framed her to escape Rand. My hunch is that RJ intended to work out the details of those two Forsaken once Rand's storyline in Arad Doman was more advanced and all mapped out. That's when I think he would have thought "I could use a Graendal scene there, to support or contradict Rand's notions in his next POV scene" and so on. A lot of the Forsaken scenes seem to play this role or something similar, and I would really not be surprised to learn that RJ created and inserted such complementary/counterpoint scenes only once the book had really taken shape

About Perrin, my feeling remains that RJ probably knew which Forsaken was involved but did not outline any specific role or scene for that character. Typically in previous books, we would never have seen that Forsaken and be left to suspect there must be one involved. Just like the Forsaken in Mat's storyline (we got clues it's either Cyndane or Moghedien, so I believe the other not targeting Mat was targeting Perrin). I think to use Graendal against Perrin is a "hole" RJ left that Brandon chose to fill that way. It's not a very logical call for Moridin to make. He knows Perrin is a dreamer, and he gives a device to Graendal that interacts with TAR. It doesn't make sense for him not to keep his secrets for one thing (ie: to reveal his toys) and for him not to use one of the two pets when they had the best skills for the job and much better chance at countering Perrin's TAR skills. It was a low risk mission from the shadows using agents perfectly suited for Moghedien.

And yeah, compelling Byar was totally unnecessary - he was heading straight for that logical resolution anyway and it cheapened that fact to add Graendal to the mix. It left me the feeling that RJ left details about Byar's attack but forcibly said nothing of his motivations he knew by rote, and Brandon thought this needed some outside and explicit motivation to work and turned this into manipulation by a Forsaken. Byar is perfectly logical through the story, it's more Asunawa who's been seen acting weirdly at times, and who very conveniently moved to remove the only man who had a bit of chance to regroup the Children and flee in front of the Seanchan (especially since he got the confirmation they had indeed invaded Tanchico right as he died).