It's beyond common sense.
They can't be made incorrect if there's nothing incorrect about them. Again, Villager is pure proof of this. He has no legitimate personality. He was given one in Smash first, where he's more dark than the original game, where has no personality at all. Just tiny reactions to events that happen.
Besides that, Wii Fit Trainer is more proof of this. Little personality to begin with. Doesn't have a "will not fight" personality. They like to teach people how to actually keep in shape. That's... it. Tell me where on earth this implies "will not fight", because it's the same proof as before. They either show a "will not fight" or they don't. If they don't, it's not an issue at all for Sakurai to even look at. He had to say no to Villager before because he believed the character would not fight. He changed his mind, but unlike Isabelle, it's a hard fact he was a blank-slate. This is a key factor that needs to be considered. And being she's already treated as a "will not fight" character as present in Smash alone, this means adding her in will require more than an echo alone. Or even a semi-clone. He already has a personality to reflect that is in Smash. So as I said below, accidental fighter or terraforming at least could reflect her better.
Nothing, because he barely goes against personalities to begin with. Besides that, he already made one for Isabelle, being a "will not fight" character. Why would he change the role he already gave her just for the sake of it? It's not going to be a case where "suddenly she'll fight". It'll be a case where he finds an interesting way for her to battle without acting violent/having violent tendencies in order to keep what he already established. The idea of her being an accidental fighter or terraforming fighter sounds remotely possible. An echo fighter does not.