Aran'gar was obviously reasonably competent at using compulsion, given that she was able to use compulsion on Ramshalan without totally wrecking his mind. And that was with just few moments of warning and few minutes to work with.

With weeks and months to work on Egwene, I see no reason why a slow, carefull and subtle compulsion could not have been put into place. A tiny bit one day, a little more the next and so on. Such a compulsion would likely not be noticable to those around Egwene, and even if it was, so what? So long as she did not suddenly start acting totally crazy or completely unlike herself, any changes would certainly be attributed to the stress of her new position as an Amyrlin.

The whole point here is that the compulsion would be a subtle one, and not blatant like for example Rahvins compulsion. Subtle enough that even us readers might not see it.