I don't have much of a horse in this race as I don't care about Shadow nor do I think he's incredibly likely, but I do care about good speculation, so I'll chip in here,
When it comes to speculating, theorycrafting, or any sort of argumentation you always need points and evidence for your stance to stand on if you want to be taken seriously. This means citing proper primary and secondary sources and interpreting them in a way that benefits your argument but also isn't too far of a reach. You were on the right track by bringing up the Sakurai interview, but you reached too far when interpreting it, and had your point immediately undermined. In the future, providing more sources to support your argument would be advised.
Second, the strength of these points will vary depending on how absolute your claim is. The claim "Shadow could possibly be one of the last 3 DLC fighters" doesn't need to prove itself nearly as much as the claim "Shadow will definitely be one of the last 3 DLC fighters." This is why my long-form speculation pieces never make definitive claims, only introduce possibilities and argue for likelihood. It means I have less to prove and I don't have to fight as hard upstream against the inevitable counterarguments I'll run into. That being said, you should always consider prominent counterarguments and make sure to either address or acknowledge them, lest your argument be completely undermined.
The major problem you seem to be running into is that you'll take one piece of evidence, twist it to confirm that Shadow is absolutely, 100% getting into Smash, and then immediately get your argument toppled by the most basic of counterarguments. You continue to do this, and each time you do people take you less seriously because it's essentially the Boy crying Wolf (or in this case, the fan crying Shadow). I would suggest that in the future you synthesize evidence from multiple points, be realistic with your arguments, and ensure that you address prominent counterarguments–of which you'll face many.
The argument for Shadow to be in Smash is, at this point, incredibly difficult. I don't even know if I could make a convincing piece arguing for his likelihood because by all known precedent he is disconfirmed, and the case for an Assist Trophy Upgrade is much stronger for a character such as Waluigi–who has outrageous fan demand–or Alucard–who we know Sakurai almost included over the Belmonts. I don't envy your position, because you have a lot of work to do to convince people that Shadow is even remotely likely, let alone confirmed.