Hi, Pam. I am here quite a bit and Bob has never been, to the best of my knowledge. But there are quite a few knowledgeable people who post here often. Sometimes we don't agree and it will be up to you to determine who makes the most sense. But we do try to give you the best of our experience or lead you to a place to find your answer.
I would not put poly on brick close to a fireplace. I doubt that it would burn but I think that it would char enough to be much worse than the cleaning you are doing now. And removing any coating from brick is not fun. I really suggest that you do not take a chance.