Lol - an officer has to be able to do the job of an officer. How can he give orders if he has no way to communicate?
Scotty understands him just fine.
We don't yet really know how well Keenser interacts with other humans. But Keenser can
speak when he needs to.
Do you recall Naraht the horta? His Mum can't speak in TOS, but she can communicate with Spock. Naraht uses a "voder" in the novels and comics, and moves from ensign to lieutenant. Another horta uses mechanical limbs in the comic "A Rolling Stone Gathers No Nanoprobes". In the novel "Dyson Sphere", Starfleet has a whole starship full of horta Starfleet crew.
I believe they do state what ship Gaila is on though.
No, they definitely don't. In the bonus DVD footage, Kirk assumes another
Orion is Gaila, so it's quite possible that he's seen Gaila's name on the crew manifest, and therefore is expecting to encounter her but earlier in the actual movie we see Gaila's delight at the news of her posting, but not
the name of her ship.
We checked all this
when fans started saying, in 2009, that Gaila was definitely dead. It's just as likely she's still with us.