Is he willfully blind, or has there been a spell (intentional or not) placed upon him? Maybe to possibly protect Merlin initially and this is how it is manifested now? I could almost see Gaius and/or Merlin trying something in the early first season timeline but, with the whole lack of focus and/or practice he had at the time, it went wrong.

Leon so totally lives. *nods* Arthur needs at least one knight who is not some random redshirt blindly following orders.

The armour thing had to have been an oversight; it just does not make sense. Yeah, they want to stress Merlin has other ways of protecting himself and they want to show the whole warrior vs not-so-much thing, but it really stood out to me.