I think I might re-do it like this:
Requirements: Swords A, Reason A, Level 30 (Master Seal)
Proficiencies: Swords (+3), Reason (+3)
Innate Abilities: Swordfaire, Black Magic Uses x4, Apotrope (cuts all magical damage dealt to the unit in half/final calculation step (stacks with Aegis activation/etc.))
Base Stats: HP 32|Str 16|Mag 16|Dex 14|Spd 16|Lck 12|Def 14|Res 20
Growth Rate Mods: HP +20%, Str +15%, Mag +15%, Res +15%, Cha +5%
In-Class Stat Bonuses: HP +2, Str +3, Mag +3, Spd +2, Res +5
Movement: 6, Infantry/Mage-typed (no sand penalty, etc.)
Mastery Ability: Warding Blow (unchanged)
Mastery Combat Art: Mistwalker (Swords; Mt +5, Hit +20, Crit +0, Rng 1~3, Dur Cost 5; Stops target from moving or using magic for 1 turn)
Favored Users: Byleth*, Dorothea, Felix, Ignatz (maybes include Edelgard, Annette, Lorenz, Marianne, and Hanneman)
I'd absolutely expect this unit to carry a Levin Sword+ and use it as their preferred weapon/primary method of offense most of the time. Their black magic charges are supplementary, for when the character's Levin Sword is about to break or when the side-grade benefits of just casting normally outweigh using it (for example, Wind is far more accurate and lighter, and a Mortal Savant would have a whopping 48 charges of the spell to play with; having 4 Meteor charges is probably reason enough to use the class all by itself, Tomefaire or not; Excalibur has the anti-flier attribute, an enemy Lance-wielder might have Swordbreaker assigned, etc.). The Spd penalty's gone; that was just a head-scratcher, IMO, not sure why that was there since the class isn't mounted.
The primary role of the class, for me, is being an anti-mage. Apotrope flatly spits in the face of magic attacks, even Luna Λ. Just certing for MS automatically brings the user's Res up to 20 (25 while in the class), minimum. It has the highest Res growth modifier of any player class in the game, it still learns Warding Blow as before, and now it also gets the Mistwalker CA, designed to allow the MS to pretty much freeze a foe in place and stop them from casting anything even if the unit using it lacks personal CAs for those purposes (also handy for catching up to something with better mobility, like a Dark/Holy Knight).
Falcon Knight is a decent anti-mage already and more mobile/speedy, that's fact. But, the non-protag favored users for MS either have a Flying weakness (Dorothea) or are dudes (Felix and Ignatz), and Falcon Knights do not mixed-attack as well as an MS due to the Arrow of Indra being...very, very limited. Not having any magic access is also a bit of a bummer (MS would still gets the unit's Faith spells if they've bothered with that skill, just like before).
The main thing that bugs me about this alteration to MS is the still-present Black/Dark Magic divide, which screws Lysithea over (Hubert is a very limited-availability unit and would rather be a Dark Knight, anyway). She has Gremory, though, so I guess it wouldn't be that big of a deal. I also half-wonder if MS got over-nerfed to compensate for Byleth's Enlightened One mid-development, as admittedly this 'magic swordsman' variant's probably a stronger overall class (then again, Byleth gets EO for free, and TBH that one feels like an Advanced class to me, not Master).