“Even if he held it was a safek, we clearly don’t pasken that way, as we are satisfied with one, and we say a brachah.”
We are not satisfied with one. We just don’t have any other choice. Many poskim say the modern day “kohen” should give the money back, even lechatchila, because it is very likely he is not truly entitled to the money as he is very likely not a true kohen. Today, it is even vastly worse than it was 200 hundred years ago regarding who is and who is not a kohen with the entire BT movement- some people mistakenly think they can just look at a gravestone and if engraved within them are are the iconic Kohen hands will say “great, I’m a kohen” and they mistakenly tell their kids this and on goes the cycle.
Regarding saying the bracha, this is no problem at all. The poskim ask how can it be that modern day “kohanim” are allowed to say a bracha since they are very likely not true kohanim. We say that safek brachos lehakel does not apply to a birchas mitzvah.
Anyway, I will say it again, this has next to nothing to do with the asker’s question and is not the right place to address this topic.