Yeah, the “for x and I” is definitely a hypercorrection. But I don’t think the original “me and x” is the result of prescriptivism - I’m not sure what correction would cause it. It’s interesting because French has a very similar phenomenon with its emphatic pronouns (e.g. “Ma mère et moi, nous faisons…” “My mother and me, we do…”) In any case, it would be interesting to see how things develop over the next few hundred years