[magician4]

I appreciate your attempt, but in my opinion the problem goes deeper than just arranging some values for "activity"

what would pH  "mean" when there's no water left to show such an pH (exclusively (!) defined in aqueous solutions)?

you would have to start with a complete fresh definition of meaning, and for a solution as described above in AcOH, which would have not much in common with systems that show a huge excess of water with respect to particles solved therein

that's why any formal calculation of "pH" here becomes meaningless, I think

regards

Ingo