Totally agree that Lopeti is an eight and not a lock. We would lose too much in the lineout jumping stakes if we were to pick him in the second row. In any case, McCalman and Holloway provide defence, go forward and jumping ability off the bench in the context of the current back row we field.
I'm of the Bakkies school when it comes to the second row pairing: you pick an athlete for the lineout and ball running in the loose and the rock shifter for the hard dirty stuff at the breakdown. Both players need to be able to scrummage. Simmons will be playing, no doubt about it (barring injury or suspension). What remains is who to partner with him. I'd say it will come down to Carter, Arnold or Coleman, with Carter my front runner.