Edit - this all assumes civil court, not criminal since the question does not mention cops.
The problem you describe sounds procedural. It does not sound like any spoliation has occurred. Assuming you can get a hash match the fact the computers are "unsealed" for a period of time is not going to matter.
But assuming you don't get a hash match, the evidence can still be admitted. The attorney will call the forensic expert who will lay the foundation for the evidence. Stating what it is and where it was found etc. On cross examination the other side can ask about the hash problem but if the expert can establish that the evidence as shown is accurate then the chain of custody might not carry much weight. In other words the evidence will be admitted, it's up to the parties to argue about how to weight it.
It sounds like the concern is that the owner was not present when the hashes were calculated. I don't see what this gets you. It's rare for a party to hang around while forensic engineers do their work. As for whether he wrote down, copied and pasted, or photographed the hashes this doesn't get you much either. The hashes exist to confirm that the working copy is the same as the original forensic image. As I said at the beginning, matching hashes is all that is needed for verification and a photo can be used to support foundation.
If something was promised but not given, the judge might order the thing, but even then what does it get the party here? All that will happen is you'll confirm that nothing was spoliated or that if it was then you can argue about weight.
As for cases, those are going to be hard to find. It's rare that chain of custody impacts the outcome of a case. To the extent that electronic discovery makes it into an order or opinion it's usually the result of a sanctions motion. So start there. But even then the best facts can be found in the motions and pleadings which requires some time on PACER.
If you are going to challenge, make sure the lawyer knows what he or she is doing. You don't want the George Zimmerman Twitter guy.