eh this "plagiarism" framing is overreaching there were two proofs in the paper: countability of algebraic numbers and uncountability of reals countability of algebraic numbers is a rather trivial induction on countability of rationals/pairs of numbers, which Cantor already knew about Cantor himself did prove uncountability of real numbers; Dedekind just helped him clean the proof up to me it seems like Dedekind's as…
As you mention: Dedekind stopped corresponding with him after the publication, but also began keeping a copy of every letter he sent to Cantor.
Sure it's circumstantial, but it's exactly what you would do when you're the victim of a plagiarist.
In my eyes the burden of proof has been met.