Scheme to verify AES key was correctly encrypted for public key recipient
This would be possible by using textbook RSA, where the ciphertext is deterministic


You have the right idea there, that a deterministic scheme is the way to go. However note that every scheme is deterministic if you explicitly fix the randomness used.

So suppose you encrypt $k$ for $C$ as $c=\operatorname{Enc}_{\text{pk}_C}(k;r)$ using $C$'s public key and the randomness $r$. Now for the arbitration you give $k$ to $B$ who checks that the $k$ actually is the correct one. Then you have $C$ send the received $c$ (call it $c_C$) to $B$ and have $A$ send the sent $c$ (call it $c_A$) along with the randomness $r$ used for the encryption to $B$. You could possibly encrypt the randomness here using $B$'s public key. Then $B$ re-computes $c'=\operatorname{Enc}_{\text{pk}_C}(k;r)$ using the received randomness and its own known $k$ and checks whether $c'$ matches $c_A$ and $c_B$. If it matches at least $c_B$ then $A$ was honest (because then $B$ got the right key). If it only matches $c_A$ then either of $A$ or $B$ is lying but we don't know which, because $B$ could have produced its own ciphertext and sent it to $B$ or $A$ could have produced a correct ciphertext for $B$ but sent a bad one to $C$. I don't think there is a way to resolve this issue and find out who lied at the end.