I am less familiar with A3 rules, but shouldn't original route crediting be what they should do? It seems A3 is of the opinion that pax himself cancelled and that with that, the whole ticket is void. You present it as a case of denied boarding with a reinstated ticket (on the same flight I guess) but still with the ATH-MUC flight invalidated since I guess the outbound was now another ticket. Did you have to pay for reinstating? Where there different PNRs for the reinstated flight and original return?