The problem with Fire Emblem and Swords is not a Smash thing, it's something that's carried from the franchise itself.
While the games are all about different kinds of weapons and classes, the protagonist are almost always the same good guy using a sword. All the Lords that have a weapon that's not a sword are either stuck with another Lord in it's game (Hector, Ephraim, Micaiah), or the non-sword weapon is just a secondary thing, with their main weapon still being a sword (Alm, Lyn, Celica, etc.).
The first one is probably the biggest problem of the two, because it creates this very awkward situation where you either add one half or the game they're representing, which would be pretty awkward and somewhat dissapointing for fans of the other character, or you add both the non-sword user alongside their respective sword-user companion. And that would mean adding two more Fire Emblem characters for the Smash fandom to complain about the FE pandering.
Honestly, the only solution at this point would be to add a villain, since those tend to not use a sword since they are evil wizards or dragons. But then again, it creates this awkward scenario where you have to ask: Which one? And why this one over the other one? Not to mention that said villain should probaby be from a game that already has an FE rep, else it would be weird to have a villain of one game without having the respective protagonist.
Tl;dr : You can't win against Garnef when it comes down to FE representation. You either have a Sword, or no character.