It seems Austrian accepts the validity of the claim in terms of origin and destination, and the delay times involved: but is contesting it because of the cause of delay. Or perhaps "cause of delay" is the airline's initial reaction to any claim.
If the OP does not want to do his/her own investigation into what caused the delay/cancellation (and the 60-day turn-rounds in communication could be off-putting
), then I'd suggest contacting one of the companies such as EUclaim who handle claims in return for a share in the compensation. Their commercial interest in success means they'll quickly scan the claim and decide how likely it is to give a positive result.
The points of interest - and complication - are the non-EU to non-EU nature of the trip, and the problems with passport/visa validity. Perhaps OS believes the nature of the delay trumps these considerations. So it would be nice to hear how this claim goes.