Therin of Andor wrote:
The crew's surprised reaction to Ilia's arrival (especially Decker) makes far more sense if she was a last-minute replacement.
Except she'd already, presumably, been to Dr Chapel some time earlier - pre-flight medical assessments? - because Ilia "once mentioned" the significance of her Deltan headband.
UHURA (OC): Transporter system fully repaired and functioning normally, sir.
SULU: Dock signals clear, Captain.
KIRK: Reply we are holding position awaiting final crew replacements.
UHURA: Aye sir. Transporter personnel reports the Navigator Lieutenant ...Ilia. She's already aboard, and en route to the bridge, sir. She's Deltan, sir.
ILIA: Lieutenant Ilia reporting for duty, sir.
KIRK: Welcome aboard, Lieutenant.
Sounds like Ilia is a last minute crew replacement who goes directly from the transporter room to the bridge, to me. I agree, it's plausible that she's replacing the female killed with Commander Sonak. As for when Christine heard Ilia mention wearing her headband?
CHAPEL (on viewscreen): I remember Lieutenant Ilia once mentioning that she wore this.
There's plenty of opportunity for Ilia and Chapel to talk off screen, such as right after the wormhole, when she's on her way somewhere in the corridor and crosses paths with Decker. However, I must say that "I remember [her] once mentioning" sounds like they've talked quite a bit over an extended period of time. That would kinda suggest that they've served together before the film starts. But when and where? Not on the Enterprise.
Kirk doesn't seem to know her, and Decker hasn't seen her since he was on Delta IV. Perhaps Chapel wasn't continuously on the Enterprise
after Kirk left.