I don't think anyone else is claiming that 'walk-through' could be plural of itself.

There are some people who just don't get instructions. So multiple walks are required. Which would make it "walks thorough"

what is said verbally was common parlance (can parlance be written?). Soooooooo would you prefer "what is uttered.......?" - clunky if consistent and don't get me started on "verbalised".