Totally inspired by the game Furi. He's actually a character with an important role in other stories of mine, so I'm glad he was interesting.
You know it's funny that you mention that. There are some subtle things that you just can't teach and I was driving myself crazy about whether I should restore Mendez's arm so quickly or not. Seems like a simple thing, right? Nope. If lopped off arms are so easily restored then it affects the entire canon of the story. What are the rules, what are the limits? I curb stomp Naruto for Edo Tensei and Bleach for its Quincies but now I see how easy it is to just say 'meh, this is how it works' without considering the repurcussions. I have some rules in my head already of how far I can go with anything, but it's pretty funny. And I totally missed some crucial character design rule on that front, haha.
Er... As for the canonical reason why he's healed so fast see Chapter 4. In the words of his own sister 'Healthy as a god and dumb as a dog.' Even for a swordsman, he heals pretty fast.
I'm curious, did you understand what happened with the bridge scene? Art is wonky but even if it is I need the reader to know what's going on.