Only thing missing is the explanation how it is possible that a fork happens and the correct nodes are split between both of them.
However, I did not know that Graphene blockchains use a Pos/PBFT hybrid. Are you sure about that? Do they gather two quorums? Is there a view change algorithm?
Additionally, there are already protocols that scale traditional consensus linearly (HotStuff, Byzcoin, etc) that could scale easily too much bigger numbers of witnesses.