Still, Chekov Prime never struck me as more than a competent science officer -- hardly a Wesley Crusheresque child prodigy.
I'd agree with that, though as Chekov branched out into other things (tactical and security) as his career progressed, it probably wasn't necessary for him to be anything other than competent as a science officer. Spock was one of the Federation's most accomplished scientists, and the Reliant
had Beach manning the post. It wasn't necessary for Chekov to wear more than one or two hats at a time.
As an aside, there's another TWOK plot-hole for you, Christopher. Why would Reliant allow both her captain and first-officer to beam to the surface of Ceti Alpha V? I know the plot reason, but wouldn't Beach have been a better choice?