I mean, I'd call most of the points provided as circumstantial evidence. Never said that they don't hold any merit, but I just like to keep my expectations low unless something really, REALLY, concrete pops up.
In no particular order:
Indie Crossovers - Shantae not being in games like RoA or Blade Strangers could be as simple as WF not being approached by those particular devs. Shovel Knight's immense popularity is what probably gets him to be requested so frequently as a guest. Shantae's certainly popular, but as much as a dedicated cult following as she's got, SK's probably just more recognizable (despite not being around as long) simply because his game was such a big hit from the get go. Not to mention the continual updates and aforementioned crossovers have kept him in the limelight.
Tweeting Jammie Selfies - These are the only ones that I think can be taken as proper hints especially so with the dialogue in Jammies. My only hangup here is that while it might hint at her being in. It could still very well be just as an AT like SK. Some would argue that hyping her up only to be an AT or more minor role wouldn't make sense but I think that the amount of enthusiasm WF has for Smash would say otherwise. Even if it's not as a character, I'm sure they'd just be stoked to have their character in Smash period.
Chairs - ...
Again, being a downer wasn't my intention, I just don't see her as a lock just yet.