Releasing vows about wearing pants

She did not make a vows revocation on the eve of the R”H. So that does not apply.
Besides the fact that this is not a clarification of a mistake but a desire to make things easier for herself, will she have to make a vows revocation before the Lay Court? Or can she do something else?

Yes, it is desirable that they have some knowledge of the laws of vows.

Her motivation is a combination of the fact that she thinks pants are modest and that she wants to make it easier for herself. Does this fall under the definition of being wrong in the law and then she won't need to have her vows revoked?

In the same vein, can a woman appoint a messenger to have her vows revoked?

So maybe it's worth doing. And I'm still not sure it's necessary since she didn't really take a vow/swear and in fact she wore a skirt and not pants, which is a passive rarity. It's hard to say that she didn't wear pants three times, because every woman sometimes doesn't wear pants. On the other hand, according to your description, there are indications that she did it intentionally, meaning that she had the intention of accepting it upon herself, and perhaps even said it in practice.
A messenger for the divorce is a Rishonim dispute. The Shulchan Ar-Ra'd rules that it is not done, but the husband can be a messenger.