I'd assume that a couple of second rowers are likely to be pushed up to the front row to accommodate Scott being out and Myles being shit. Based on incumbency and common sense (see my Myles comment for clarification) I'll take a guess. Bolded the ones I talk about afterwards.
18th man: O'Neill
Thaiday and Papalli will fill two of those spots, and both can be pushed up into the front row if needed. Maybe push one up front, the other from the bench with Guerra sliding into the second row next to Gillett.
That would leave two bench spaces and a centre spot. Cooper played game 3 last year and has a killer combo with Thurston so he slides into one bench spot. From there I think they need a bit of size and a proper prop so if fit Napa could be the last bench spot.
Centre spot is tough. O'Neill/Chamber/Gagai are all right hand centres. You would have to think that Chambers is the guy to come in, but not necessarily to left centre as O'Neill was only there last year because of injury. Perhaps the wildcard though could be Gagai to left centre and Slater/Holmes to the wing? I think Boyd has the fullback slot locked up but don't discount either of those two.