Even if the crew is changed at Dhn, still another crew change will take place at Asn as for the upward journey, crew was given by Asn and not Dhn. Actually I strongly believe that a single crew took the charge of the train from Ddu to Asn, it's a very common practice for diverted trains, LP, Alps get overtime for these works as intermediate zone don't provide fresh crew members in 99% cases.
And regarding 8hrs working work, crew of 12305/06 takes the train directly from Hwh to Pnbe and vice versa which has a working time of 7-7.5 hrs, so it's very much possible to have a work of 8hrs or more in special cases