A number of topics to cover here...
This makes sense. Protocol 253 is used by our McAfee Network Driver (aka MFEND). This is used to determine what nodes are available for load distribution. So if another MWG in the network comes online, we will add it to the ProxyHA cluster.
We also use VRRP for determining ownership of the VIP (for failover).
As for the interface related questions, in general I would advise disabling any interfaces that are not in use. It wouldnt be good if somehow the interface picked up an IP.