I feel like you're disregarding a lot of fan rules that people made up during Season 1 speculation. Remember when people thought no Western owned characters would happen at all just because it never happened before? Remember how people thought guns were still an issue and disregarded Bayonetta's guns as just "magical guns"? These two topics were definitely talked about a lot before Joker and Banjo got revealed respectively.
And if we're gonna excuse Byleth's exclusion that it didn't break the "All new worlds" statement because Fodlan isn't in the game even though he is another Fire Emblem character, then you could say the same for Waluigi on the pretense that there isn't a Mario Party world represented in Smash so he could bring that in with a fighter pack. Actually why are we even regarding Reggie's statement as anything more than just a vague PR statement that meant nothing in the end? Reggie made that statement on the spot at the game awards to hype up the fighters pass.
Anyways, yeah, precedence can be used to back up your thoughts on something, but it shouldn't be used as objective evidence when it comes to speculation. Because let's be honest, every Smash game has broken some form of precedence that people relied on at one point.
I don't know that Mario Party is the equivalent of a new Fire Emblem game, mainly because Mario Party isn't a game that represents a different storyline, with different protagonists, or different stories. It's a Mario spinoff game, just like every other Mario spin-off game.
I myself always said that Astral Chain or Three Houses would be an exception if the pass wasn't all 3rd parties, mainly because Corrin was a similar add in Wii U. In addition, it's a title in a game with a new cast, new music, new stage, etc... But more than that, it simply fits the "shill" protagonist pick that we got the first time. Many Nintendo characters that are currently asked for do not fit that trend.
Now, it's not an impossibility, and it's a precedent that can be broken. But will it be broken? I'm not so sure it will when there are plenty of picks that could continue the trend of the crossover without digging into upgrades. I can't say I would be entirely disappointed, though there are picks I would be much more excited for.
I'd say that yes: Precedence works as objective evidence because it's something that has
actually happened. It doesn't mean that it rules things out going forward, but it's a valid bit to base further speculation on.
It's something far more concrete to base speculation on than a statement such as, "Well what if they change their minds?" Asking the question of, "What if I get struck by lightning?" is equally as hypothetical in this case, and unless I produce more evidence that I
will be struck by lightning, it doesn't amount to much.
At least, that's my perspective but...I'm an English teacher. That's just how I view that.
EDIT: The gun argument was definitely silly though, I totally agree on that.