Publishing a review of another person's proof
If you overtly title your piece something like "explication of X's proof of Y", and give the citation, (and do not literally copy verbatim swaths of their article) this is a completely fine thing to do. Or give it a more explanatory title, and in the text immediately say that you are simply expanding or elaborating on the proof in the original source(s).

It will absolutely not hurt your future, because the people who are "strict" about what "publication" supposedly means will ignore anything that appears only on arXiv, because, in their minds, it was not "published" at all, anyway. And, to many people, taking the trouble to digest an important thing and re-tell it for the benefit of others is a wholesome activity.