I think Kirk was completely within his legal authority as Starship Captain to maroon Khan and Marla and the gang.
Reliant's crew were talking about Ceti Alpha VI not V. They had no need to ask the computer for information about planet 5 same as 1, 2, 3 and 4
Chekov however is a problem. I offer when he was electrocuted by VGer in I, or he overdosed in adrenaline in The Deadly Years, or he was punched in the head by a Klingon in The Trouble with the Tribbles, or when he was shot in the belly in the Old West planet, he amnesiaed Khan.
Just for completion, later he had half of his brain eaten by a Ceti Alpha crab, explaining his drone-y behavior in III, and then he fell off an aircraft carrier on his head. The extensive brain damage explains why in V he got lost in a national park and why he, as a Security senior officer aboard the Enterprise, he just forgot (in VI) about basic things like automatic alarms on phaser fire inside the ship and gravity boots.
Someone should write a book titled "Chekov's Brain". I suspect it would be short.
This was a beautiful post.
