As for these things (tauta). Accusative of general reference.
One stone upon another (lithos epi lithōi). Stone upon stone (locative). Here both Mar 13:2; Mat 24:2 have epi lithon (accusative). Instead of ouk aphethēsetai (future passive) they both have ou mē aphethēi (double negative with aorist passive subjunctive). It was a shock to the disciples to hear this after the triumphal entry.