The repair shop claims that the keyboard was damaged upon arrival, and have pictures to show for it.
That doesn't seems to me so normal. Do you mean they knew this before they start the repair?
The repair shop received the keyboard in a box I guess. That means that when they opened the package, they found into that box some broken pieces of your keyboard and they saw the damage that could had probably happened by the post office.
If the damage happened from the post office, then why they didn't inform the music store directly, to start the procedure about claiming amends for the damage and also inform you about what had happened? Why did they just took some pictures as they say and they continue to the repair like nothing happened?
The above situation make me think that finally the music store made the damage before send the item to the repair shop. Then the repair shop received this damaged, they took some pictures to be safe, they talked with the store and the store told them to continue with the repair or they were trying to find a solution about how they will replace the case, so that is why they late so much. Finally when they saw that they can't find a replacement, the repair shop sent this back to the store and the store was hope that you will receive the keyboard without see the damage and then afterwards that you would going home, they would telling you stories that you broken it.
The plan B was that if you was watching the damage before you leave the store, they would claiming that the postal office did this, so they would avoid to take the responsibility too.
All the above come to my mind because I have seen a lot of these kind of stories to happening from stores that they seems serious businesses, but finally at difficult situations they try to not take responsibilities at all.
Whatever it says the law of your country, you are a European citizen and you have the same rights as me. I live in Greece and at my country we have 2 non profit organizations that they help customers to find their rights against companies, without charge any money of course, so maybe you have something like this to your country too. We have rights as European customers, so take advantage of the European laws.
I wish this nightmare will has a happy end.