Presenting an expository paper that has already been accepted by a magazine
Since you will inform everyone about the situation, there are no further ethical concerns. You need to cite the earlier work in the later one as usual, of course. And the submission to the conference may be accepted there or not, depending on the judgements of others. You are making it possible for that to be an informed judgement, so all is well.
The citation avoids any possible charge of self plagiarism.
I would also give the same answer if it were a research journal and a professional conference. If you give the impression (vocal and written) at the conference that you are presenting prior work, rather than new, it would be fine. Expository talks on existing research should raise no issues. There are ways to step over the lines of propriety, but this isn't one of them.