Thanks for the little comment! I got some bad news and good news for you. Let's start with:
The bad news:
The Amiibo data isn't credible. The one who posted that was an obsessive Daisy fan who included the same data regarding Daisy in that same post. He, Memoryman, is notorious online, and has no record of datamining. On top of that, he "debunked" the waddle dee amiibo data himself. And once again, he's not a credible source at all. So either he either A: Made both up (which I'm betting on), or B: Only "debunked" it because Daisy's in smash and therefore has no more reason to care about Dee.
The good news:
You're right about 3D's final smash, and I wouldn't normally bet on it, but since they went out of their way to include Fiora in Shulk's final smash this time around, I think there might be some merit to the argument.
Other things: His Guest Star appearance in Star Allies has some interesting wording on the pause screen. Going out of its way to talk about how Bandana Dee is different from other Waddle Dees, and saying he "deserves a prize", many speculate this is referring to his playability in Smash Bros. On top of that, in Team Kirby Clash Deluxe, he also says something among the lines of "It'd be nice if I could get a reward, too...". So who knows? It might just be pointing towards his inclusion, as we have no information so far as to what that prize could be. (As far as I know anyway.)
Another thing to consider is Sakurai warming up towards Modern Kirby. We already had Kirby's Final Smash in 4, but he also had no input on the creation of Masked Dedede (appears in Dedede's Final Smash) and Galacta Knight (as MK has a costume resembling him). So I feel like the "Sakurai is biased towards his own games" argument doesn't hold as much water as it may have had before Smash ultimate was shown at E3. I also recall hearing that one of Kirby's victory animations is based off Return to Dreamland, though I never really bothered to check this myself.
On top of that: In
this interview Sakurai states that he looks towards the Ballot for character picks for Ultimate. And with how big of a deal Kirby is in Japan, and Bandana Dee being a VERY high request in Japan, I think his chances are as high as they could ever be, ESPECIALLY when considering Kirby and the Rainbow Curse got released only MONTHS in Japan and NA before the Smash Ballot started, where Dee was the only other playable character than Kirby, it's safe to assume he's got plenty of Ballot votes when taking his few other roles into consideration.
One last little thing: Considering his aforementioned roles in modern Kirby, being pretty much a main character and all, I feel like it's safe to say he'd be an assist trophy at minimum, but so far, we haven't seen him. Hell, KAWASAKI was revealed to be a/the new Kirby AT.
And while slots may be extremely limited this time around, I personally feel like Dee's got very little competition when it comes to first party unique newcomers. (And I don't think we'll be getting as many third parties as some others think. One more unique third party at most is my guess.)
Though it's always wise to keep your expectations in check. I'm personally convinced he'll be in, but you never really know until he's official revealed, or deconfirmed. I myself as am hopeful as I'll ever be.