Appreciate you putting time into the analysis.
Seeing improvement later into the game is a good indication of his fitness level and also his mental application which are two things that should be held in high regard. His Adelaide game was a very similar trend.
What it also indicates is, that when the sting went out of the game and it opened up a little McKenna was able to find a little more time and space to make good decisions. He has performed very well at VFL for this very reason, its just a slower brand of football.
Maybe a move up to the wing may be more suitable for him at least until he feels more comfortable at the level and then he can move back again.
The wing position may pose other problems though because you probably have a good understanding of when and where to run.
All in all I think he's on the right track and needs to be persisted with.