Er...yes. I have no idea why, but I constantly - CONSTANTLY - get Peter and Mike mixed up. Even while watching the show or their movie.
The 'Mike - Hat; Peter - no hat' thing SHOULD make it clear, but somehow I always manage to swap them in my head.
I didn't see the Walter Koenig resemblance
It's his mouth - Koenig has a rather distinctive mouth, and Davy's sporting it here. (Add in the hair, which was, as you said, modelled on Davy, and he looks almost exactly like Chekov...before Koenig grew his own hair out and stopped wearing the wig.)