Yes, a "set of elements" is potentially ambiguous, but in my opinion, what you have here is a TOC and thus all of the links are elements of the TOC. So I think it might be a reach to consider each nested list of links as a separate set.

As for as whether you can do this, if you look at the ARIA 1.3 docs, it says that "Authors SHOULD only mark one element in a set of elements as current with aria-current." That "should" is important. It means that it is strongly encouraged and considered a best practice, but it's not "illegal". So technically, it is permitted to have multiple aria-current attributes in a set and thus you "can" do it, but whether you should is another question.

Most accessibility professionals will tell you that the only way you can really determine what works best is to test with your users. My suggestion would be to start out as simple as possible and only make it more complex if you find out through feedback/testing that you need to do more. Thus, I would recommend you start with only one aria-current at a time and put it on the link for the actual section that is currently in view.