Then again, we have no reason to think that Decker's crew made an appearance in the movie. Decker's ship wasn't ready for a launch - so it's quite likely that his crew wasn't, either. When Kirk pushed for a faster launch, he was already placing himself on the center seat, so there would have been no need for calling in Decker's XO; Decker himself would be demoted to that position soon enough.
McCoy wasn't supposed to be in Decker's crew, either, and we aren't worrying about who and where Decker's own CMO was (might have been Lieutenant Chapel, might have been somebody else). Many such people were no doubt left ashore, as they would have counted on not being needed for several weeks at least; they could well have been on other ships or planets at the time, rather than wasting their acknowledged talents in idle waiting.
Sonak was Kirk's choice for CSO, but for serving under Decker - the fact that he would be serving under Kirk comes as a surprise to the Vulcan. Certainly Sulu, Uhura and Scotty appear to have been part of Decker's crew from the very start, although we
could argue they were only part of his refit team and would otherwise have departed prior to the ship's launch.
The film lists the deceased science officer as a lt. commander so that really shouldn't be the case.
LtCmdr is a valid rank for Starfleet starship XO, as exemplified by LtCmdr Cavit and a number of nameless extras. However, Sonak was never identified as LtCmdr. Kirk always addresses him as "Commander", and while that may be ambiguous, his braid indeed is that of a full Commander.
George Kirk was referred to as "Lieutenant," if I remember right, but is wearing what appears to be rank braids of a Lt Commander.
George Kirk is not given any rank in dialogue while he's alive; Robau calls him "Mister". But Pike calls Jim Kirk's dad "Lieutenant George Kirk" at his face after the bar brawl. We glimpse George's braid here:
http://movies.trekcore.com/gallery/albums/xihd/trekxihd0155.jpg
Two solid stripes, which in TOS would be full Commander. But one stripe is thicker than the other, which leaves us wondering. Here is Robau's braid:
http://movies.trekcore.com/gallery/albums/xihd/trekxihd0119.jpg
Looks like three stripes, when TOS had Captain at two solid stripes and one dashed. So, is a narrow stripe the same as dashed? If one of Robau's is narrower than the other two, then the system is more or less the same as in TOS (and in the 2250s scenes of the movie), and George Kirk is a Lieutenant Commander. If not, then all bets are off.
Timo Saloniemi