I think that either Chekov or Terrell upon learning the destruction of the Enterprise would have been able to fight off the eels similarly to what happened in the Genesis cave, (watching ST2 as an adult, I always wondered if Terrell hadn't turned the phaser on himself, would he simply have collapsed and had the eel leave his body like it did with Chekov?) If they recovered in an isolated area of the Reliant, Khan wouldn't know they were free from the eels control. Then they could if anything, set the ship to destruct...killing themselves but knowing for sure they stopped Khan.
I think TWOK reasonably establishes that without medical assistance (and possibly without the kind of talented medical assistance that someone like McCoy can offer), the odds of surviving a Ceti Eel attack are negligible. They did kill a good number of Khan's people, so I don't think Chekov or Terrell would have recovered on their own unless they somehow made it to Sickbay or found a sympathetic person among Khan's crew, and possibly not even then.