Erm... I haven't tried to prove that he'll be a stage hazard. My unshakable faith in Ridley makes me believe the opposite; I'm still on the playable Ridley bandwagon. What I've been trying to prove is merely this: the character that the quote hints may "appear at any second" is Ridley. To support this; I've brought up the similarities in the pictured stage to Ridley's battle stage and the nature of the quote itself.
Now that we have that misconception out of the way, I do stand by my claim: the "trauma" that Sakurai references can only be Ridley! You've successfully proved that the quote (specifically the words "trauma" and "enemy") can refer to Mother Brain in addition to Ridley. Even so, the image and the quote together form the decisive evidence that Ridley is the one Sakurai is talking about and perhaps teasing.
I'll start with your whole "hole" business. The stage itself looks very much like the one from Other M. They may not match up in EVERY respect, but they do closely resemble each other. A few details are different, but such is only to be expected when adopting an area to a completely different genre of game. Just look at how the world of Hyrule changes depending upon the Zelda game in question; the location is changed to meet gameplay needs.
We do not have a comparable moment relating to the Fountain of Dreams or even to New Pork City. Sakurai never mentioned anything about enemies appearing when he was unveiling either stage. If he had done so for Fountain of Dreams or New Pork City, it would have been a completely different story; you'd be correct in saying that the Miiverse photo and description do not concern Ridley. As of now, however, we must acknowledge that this situation is unprecedented; Sakurai hasn't ambiguously mentioned an enemy appearing on a stage with a choice of enemy that is obvious.
My Closing Statement:
Sakurai describes what can only be one of two characters: Ridley or Mother Brain. Ridley is the character who more closely fits the description of "Samus's Trauma," suggesting that he is indeed the one who will appear.
Because of the picture Sakurai provided along with his description, which depicts an area closely related to Ridley and completely foreign to Mother Brain, we can only assume that the character he wants us to envision is Ridley. Why would he post a picture of Ridley's battlefield if he wanted us to think of Mother Brain? It makes absolutely no sense.
The only case of a "boss" character appearing on a stage where they do not belong is New Pork City. However, this is the exception and not the rule; additionally, Sakurai did not taunt about an incoming enemy then, meaning that the current situation is different.
The current situation, Ladies and Gentlemen, is Ridley. Ridley is coming; hazard or playable, we will know "any second."
This was a great debate, really. But I feel as if we've drawn it out long enough. I don't think I managed to prove my point beyond all doubt as I should have, but I do think I made a few things clear. I made it pretty obvious that it refers to Ridley, even if my arguments may not be enough to sway a court of law.
As of now, I'm going to go and play some Ace Attorney. Adieu.