If you haven't been convinced as of current, nothing I could possibly say would influence your decision. But you are correct, I NEVER assert that a character is definitively in the game unless I am truly confident. The only character that has ever been afforded that respect was Mewtwo.
Ridley is the most prominent and prevalent Metroid character aside from Samus. There is an overt series discrepancy with Metroid, it is one of the most notable Nintendo franchises with only one representative. So Ridley IS the definitive choice for the roster in regards to Metroid. His popularity is staggering, as someone mentioned earlier, it is literally impossible for Sakurai to discern Little Mac's popularity in the West while disregarding Ridley's. Sakurai has also personally spoken on account of Ridley; that is something, in my opinion, that demonstrates an interest in the character. He is also one of the most relevant characters pertaining to the Smash series, he appears in every title.
His role as a boss character was already used in the last installment, this could conceive a precedent that entails his return as a boss character; however, I believe that Ridley has been one of the least stagnant characters in Smash. He was a background character, an opening cameo, and a boss character. Reintroducing the character as a stage-hazard is seemingly diverse; but it is slightly redundant when one considers that he is essentially a glorified background character. The most logical innovation for the character is to become included in the roster.
Sakurai's intent; it is easy for some individuals to make unsupported accusations alluding to Sakurai as a "troll." This is not the case, Sakurai, is merely attempting to market his game in an original or creative way. If you notice, the entire "mystery" around Ridley is purposefully ambiguous. As someone who is quite familiar with the structure of novels, it is blatantly overt that he is attempting to incite discussion; he is introducing us to context that is malleable. This is the same with Ridley's introduction as a silhouette. One must not ponder one what he was stating, that is propaganda 101. You must articulate what he was attempting to convey. Again, he wanted to reveal something about Ridley; the perfect opportunity to reveal a second boss character arose, he didn't insinuate anything in regards to Ridley. It was just audio over a clip. Secondly, silhouettes are notoriously used for... keeping something a secret. If Sakurai wanted to keep Ridley's fate ambiguous, he would not have stated anything in regards to Ridley (which he didn't). So it is logical to assume that had he wanted us to learn about Ridley, he would have shown us the model. Had he wanted to keep Ridley under wraps, he wouldn't have introduced the clip.
Surely, even if you are skeptical, you have to admit that the choice was odd. Why keep his boss status indeterminate and blatantly give it away at the same time? That is irrational and counter-intuitive. It is all about the theatrics, he wants to generate hype for the event 2 months away.
Let us go back to square one, don't you think we would have learned that Ridley is a boss character if he was one? We have seen dozens of Pyrosphere photos, nothing would indicate a stage-hazard, least of all Ridley. No respected developer would "taunt" fans of their product about the inclusion of their most demanded character so consistently if they never intended to add whatever it is the fans wanted. It would be much more wise to get the Ridley reveal out of the way if it was unfavorable, he is indicating to us that the reveal is something good and unexpected. Not something expected and boring. He is not ignorant enough to accumulate such labored disdain. The Direct was full of such practice. The Palutena quiz and fallacious Zero-Suit Samus deconfirmations come to mind.
Remember, it was an audio clip over an edited game-play clip that was intended to be ambiguous. Do you want to rely on that as "evidence" of his non-inclusion?
Ah, a properly composed and well thought out argument for Ridley. So refreshing to see. Now, let's see what I can make of it...
Now, you start out by pointing out Ridley's importance to the Metroid series, and the significance of the Metroid series to Nintendo. Both of these, of course, are true. Metroid is plenty deserving of having a second (or third, technically) representative in Smash, and Ridley is the obvious, and perhaps only, choice for that role at this point. Now, you also mention that Ridley is very popular in the West and that Sakurai would have to have taken notice of that. While it's rather difficult to say for sure, it would have been very difficult for Sakurai to do any sort of proper research into popular Smash characters, particularly in the West, without finding out about Ridley. As you pointed out, this is also implied by Ridley's recurring appearances in Smash, which seem to be increasing in importance over time. So, based on all of this information, the logical next step would be for Sakurai to acknowledge Ridley's popularity and importance as a character by making him into a playable character.
But then we come to Sakurai's intent. This, of course, is very difficult to determine exactly, as we can't read Sakurai's mind. That would kind of make this entire portion of Smashboards pointless if we could. The only way to actually figure out Sakurai's intent, or more specifically, his intent with Ridley, is to analyze his actions and what we've seen of the game thus far. From what I've seen so far, I've come up with two possible explanations for Sakurai's behavior towards Ridley, and what the implications of his behavior are.
1.) Sakurai is ignorant. This would be the explanation for Ridley being a stage hazard in this game. Basically, Sakurai is ignorant to what the fans really want out of Ridley and decided to make him into a boss figure once again. All the mystery he's put into the Pyrosphere is intended for a grand reveal of Ridley being a super-awesome boss, which he feels would satisfy fans. Of course, that's not what we want. Not only is Sakurai ignorant to that fact, but he's also ignorant as to how to properly reveal this. If the state of this thread wasn't indication enough, the apparent de-confirmation of Ridley has had quite the negative impact, and a good part of that comes from the fact that Ridley's fate is heavily implied, but is still ambiguous. The result is a large split in the Ridley fanbase, with some believing Ridley is out of the picture, some convinced Ridley is actually in the game, and a good number of people who just aren't sure what to make of this. Quite frankly, this is a bad marketing move on Sakurai's part, yet he remains ignorant to this fact. Or...
2.) Sakurai is actually a tactical genius. This is the explanation for Ridley being playable. As I've previously stated, the ambiguous nature of Ridley's fate has had quite the negative impact on the fanbase. A number of people who believe Ridley's dead have actually given up hope for this game, but because Ridley's fate is still not entirely known, there remains lingering hope that is labeled as denial by most. An outright de-confirmation would have been much less painful. However, this sort of bizarre reveal makes sense if Ridley is actually playable, and Sakurai is just toying with us. So, if Ridley is playable, why would Sakurai make it appear as if he isn't? To generate hype. To put it simply, a character can get a lot more hype out of their reveal if the character was considered to be unlikely and only wishful thinking. By supposedly killing off Ridley, Sakurai is ensuring that Ridley's grand reveal will generate a much larger amount of hype than Ridley would have normally generated. After all, seeing an impossible, yet beloved, character make it into the game is much more exciting than a "shoo-in" character.
So, I guess it's a matter of figuring out what Sakurai is. Is he ignorant to Ridley's popularity as a character, how much people want him in Smash, and how to properly market this game? Or is this all part of some sort of diabolical scheme? It would seem like it should logically lean towards the latter, but at the same time, Sakurai has made some other rather questionable choices with this game and its roster...
Now, all of that is just analyzing what kind of though process Sakurai has in regards to Ridley, and doesn't really touch upon the technicalities of what we've actually seen and heard of Ridley in the game already. I'm afraid I can't really say much about that; the technical details of analyzing Ridley's shadow is a bit beyond me. I guess I would say that it would be very odd for Sakurai to talk about the appearance of other boss hazards and partially show Ridley on the Pyrosphere if Ridley is actually playable and the Pyrosphere has no boss. To me, it almost seems too conspiracy-theory like to think that Sakurai purposefully partially showed off Ridley when he made that statement just to make it seem like Ridley is a boss. By just taking that statement and clip at face value, it would seem to me that Ridley will not be playable in this game.
But... Sakurai did also flat out lie about a certain character being cut...