As far as I remember I already gave Ms Melissa a Power of Attorney (PoA) related to the Proplan lawsuit.

If it works, they can use that PoA to manage the process.

If "they" is Ms. Melissa, then it should be "she." But if you're referring to her organization, then "they" is fine.

If not, I can give them another PoA.

Again, maybe "them" should be "her."

Because it is not clear when Mehmet Uzun will take over the role of Managing Director.

This should be part of the previous sentence, not its own new sentence.