specific scenario question relating to using NACL crypto_box in a protocol
Yes, the attacker does have the ability to make messages to appear to be from SenderA. In the end, the crypto_box functionality relies on a MAC, not a signature. That means that it depends on a shared secret key, rather than a private key for message authenticity. As explicitly stated by the protocol, this allows any message to be signed if you possess just one of the private keys. Because in that case you can calculate the shared secret key and - of course - any MAC generated by the shared secret key.

If you require that messages are authentic even if the key of the receiver is lost then you need to use a signature as well. Generally you would then have to use sign-then-encrypt to create a secure protocol, especially if you consider that private keys may get known by the attacker(s) of the system. Obviously the public keys need to be trusted in any scenario.