Its a MASSIVE risk to play someone back who has never played there before. I think its likely that we will replace Young and Gibcus with 2 of Miller, Kosi and Tresize. I get that you want Lefau in the team, but you are trying to find a position for him, which if we did it would likely not end well for him. He needs to play in a position that he knows.
I would select as I set out in the Team v Port Adelaide file, with Miller at full back and Broad and Grimes on either side, and with NV at CHB with Short and Rioli on either back flank.
I have Trezise in on the wing, and McIntosh on the bench ready to play a defensive roll if needed. See post 44.
If selected Miller will certainly have his hands full. I agree with you that it is preferable that anyone going to defence have considerable experience there, but it is much more important imo that such player can physically match it with his opponent, who ever that may be..
Port Adelaide has a number of very tall athletic forward types, plus Soldo.
Big fan of Trezise but he can't match it in the air with their talls, and on the form so far displayed, nor can Kosy.
So Lefau on the bench also adding size, physical strength and great endeavour. Schooled and ready to come on and play as and where requested to the best of his ability. Not a perfect solution, but gives us a chance.