Was the angel that incarnated and lived as a priest and came to be a confidante and protector to our client, in actuality a former fallen angel, and was that a karmic payback for harming her previously?

This indeed is the case and you saw this intuitively and this is why you are asking the question to receive confirmation. It is very much an interesting example of a reality that takes place many times, where a life plan is crafted to deal with such situations, and the best solution can be to have an expression in such a fashion to make amends on the part of a spirit meddler causing harm to humans. This is a special case of crossing the threshold of species and is done for the betterment of all. This is not done, as you are wondering, in the other direction as there is no purpose for humans to take on the life of an angel. There are plenty of opportunities for karmic rebalancing and repayment in the human sphere of participation. Humans by far have the more demanding life—especially under present circumstances of disconnection. So, if one needs a further challenge or needs to have a taste of their own medicine, so to speak, with the wide variety of humans and their makeup and their heritage and karmic predispositions, there are many opportunities to experience negativity of the kind they caused in making others suffer, for which they must make restitution.