Xpost from DCW because I was hoping to talk to someone about the case.
This reminds me of a cryptography puzzle
I think this one is the asymmetric key version with a three pass protocol. Gosho seems to have set the trick up so we can assume the locks are unpickable and there are no spare keys for the locks. (That doesn't mean there are no extra lock and key sets though!) Even so, there are a whole bunch of attack routes available assuming the box is not authenticated. For the sake of explaining, the sender Kasuga who puts the fruit in will be called K. Takeki, the critic with the other lock is the receiver and will be called T. The man in the middle attacker will be M.
1) The attacker could swap out the box if both sender and receiver can't instantly recognize they were sent a different box or locks than normal. If M has two spare locks, fruitlike objects, and a spare box, he could intercept the box coming from K and put his own lock on it and send it back to K, making K think T received it. K takes off his own lock thinking the lock on there currently is Ts and sends to box back where M intercepts it, opens his own lock and peeks inside the box. Meanwhile M sends his own box with his lock and fruits to T, so T thinks it is from K. The three pass is carried out with M in the middle and T winds up with M's box, thinking it is from K. If this is what happened when T opened the box to publicly reveal the contents, K would find out he had been tricked, but T wouldn't know unless K told him. The trick would not be discovered if M could fully complete the exchange with K first, so M could put the same fruits in his box to T.
2) The contents of the box don't seem to be authenticated either -- if the sender did not include a special secret item in the box that the receiver is expecting then the receiver can't tell if an attacker tampered with the contents of the box. M, using the second version the attack described above, could completely get away with it.
3) Trick box: if the box can open any other way, anyone could fiddle with it.
4) A non-trustworthy receiver or sender could collaborate with M to fix the final contents. I'm going to guess K is likely trustworthy because he will feel the brunt of the blame if cheating occurs. T's side though seems to have holes even though T himself was probably trustworthy.
To review the security details known so far, the security of the sender Kasuga's lock and key are unknown, but the receiving Takeki thinks the lock is in good working order. I think we can assume the receiver's lock is completely open to swapping because it was plonked in an easily accessible drawer. The receiver's key is locked up in a bank vault until the show and seems to have been escorted to the premises by assumed trustworthy bankers. Anyone who could tamper with the vault or direct the bankers could switch the receiver's key. There is a brief window during the key hand-off where the person passing the key to Osamu Kenzaki could pull a sleight of hand to swap the keys. I think we can assume Osamu is trustworthy as an unlocker because he is a recurring character. The three-pass exchange itself seems to be fairly lax, suggesting that M messing with the exchanges is a viable option.
The sender Kasuga will likely take the brunt of the suspicion for the murder given the situation so far, and seeing how his reputation was already being doubted, I think he wouldn't have chosen this method as long as this crime was premeditated. Taruoka, as the one doing the handoff and directing the others (maybe even the bankers?) is in the best manipulator position. I think I'm most suspicious of him for now. Furuya had access to the receiver's lock and was not watched, so he had opportunity, just not quite as much as Taruoka who handled the key too.
