When was Aran'gar punished? Forgive me but I cannot recall right now where it fits in the timeline? Did it happen before or after Aran'gar was exposed by that visiting Asha'man and had to flee?

As for using compulsion, we learned in TGS (and from Sammael in LoC) that blatant compulsion that totally rewrites the personality of the victim leaves clear signs that others can see. Not to mention that Nynave has already demonstrated that a sufficiently skilled healer can detect compulsion. If Egwene suddenly started acting strangely, skilled yellow ajah healers would certainly have examined her and Aran'gar would have been exposed.

So if there is compulsion, it would have to be very subtle work, working with Egwenes mind to enhance some pre-existing aspects and weakening others. Even that might be usefull to the shadow though. A subtle compulsion to strenghten Egwenes fear and distrust of the Dragon Reborn could be the difference between success and failure in the meeting at Fields of Merrilor.

I would also presume that compulsion of that sort (subtle and carefull) is harder to fight off rather than blatant compulsion (such as Moghedien erasing memories from Nynaeve). I don't think any of the sisters Verin compelled ever broke free?