Before we dive too far into homebrew, let's refocus the question: You want a good representation of Morgan.
So, then, we must ask: How good of a swordman is Morgan when you consider his 'honed instincts' derived from his earth magic ?
Shiro is listed at Fantastic which, according to the ladder is:
Š Fantastic (+6) and above skirt the boundaries of natural human capacity.
He's +7 to block. But he's supposed to be the best swordman alive(RIP Shiro)
So, maybe you put Morgan at +4 or +5 (Great or Superb)
Š Great (+4) and Superb (+5) indicate veteran or masterful capacity, or the combination of extreme talent and good training.
So the question is this: How good a swordsman is Morgan without his magic? With his buffed instincts, If you add specializations, we can quickly add +3 or 4 shifts. Which which puts his 'mortal' skill at +1 or +2.
According to the ladder, a +2 skill is:
Fair (+2) and Good (+3) indicate journeyman or “professional” capacity, or a nearly inhuman degree of talent.
If the homebrew stunt/power pushes his skill any higher, then he's only a novice swordsman without his earth magic.
I think he's a very skilled swordsman. Like, +3 to +5. I think 'honed instincts' is going to net you a +1 or 2 max, which puts him on par with Shiro.(and I feel that's generous) So, if it was me making the stunt or power, I'd keep that range in mind.
If you want to add more, then Morgan is either a very poor swordsman who relies entirely on his magic for his skill or he's an expert/proffessional swordsman whose supernatural ability makes him better than Shiro.
Whatever you decide, your stunt or power should reflect that.