A) a Tor remote node is not necessary, clearnet nodes work too. But your privacy will be a little bit better when using a Tor node running as hidden service due to the fact that your requests won't leave the Tor network in this case.

B) The only difference using Whonix is that it will prevent your IP address from leaking (which is a great improvement in my opinion). For Monero remote node specific privacy risks (malicious node operator) nothing is different when using Whonix though.