Can I redeem an Austrian PSN card in the German PSN store?
Or can I set the shop to Austria, redeem the card and set Dan back to German?
As far as I know, no. Although the EU, although the euro is not possible. It doesn't work with AppStore cards either. So your account must be Austria or German and the card must also have been bought in either A or D and be intended for the same regional account. So card from A for account from A or card D for account D.