> But in this case alumni is in common use in that way and has been for some time.

I dispute that. Googling doesn't turn up many examples of official usage of alumni in the singular.

I also disagree with your second claim, which is a projection. When I said "common or not", I meant that frequency in itself does not automatically make something correct, and I gave reasons why in a follow-up comment. I didn't claim that usage was common.

I'm not trying to be difficult, but alumni really is plural. Using alumni as singular is not correct.