Other mistake: In the Briefing Room, while Kirk is feeling the effects of the disease, Spock tells Scotty that he will call the restart formula in from the bridge. But just a moment later, Kirk enters the Bridge, and Spock is in the Engine Room working on the settings and formula with Scotty.
Other mistake: After Scotty makes his remarks to Spock, he raises his phaser. The sparks start up before Scotty levels his phaser.
Other mistake: In the Transporter room, as the Decontamination field is used, the shuttered lighting casts not only on Spock and Joe, but on the walls surrounding the chamber.
Chosen answer: If they're in orbit, they're being pulled along by the planet's gravity well, therefore, impulse engines would only be used for minor corrections and would be "on standby" while in orbit, but not active. (Like keeping your car idling without revving the engine and creating plumes of exhaust).
Captain Defenestrator
Thank you for the info.
Movie Nut