The point of introducing Ridley as a silhouette wasn't necessarily intended as disguising what form Ridley will take, but just disguising Ridley. Yes, we here as analysts will understand immediately what's being hinted, however, a good portion of the people watching the direct wouldn't. There are gamers who have never even played a Metroid game, there are young kids who don't know who he is yet. In my opinion, trying to argue otherwise is an over-analysis.
1. How can you judge the size of a shadow based on it's object if you don't know the source of the light? When you're walking down a street late at night and pass by a street lamp, your shadow can extend three times your body length. That doesn't mean you're that size. The size of the shadow is completely irrelevant.
2. I don't see how that's a relevant point. Not only do we know nothing about how Ridley would act on the stage if he were a boss, we hardly know anything about the mechanics of bosses on stages as hazards at all. There's so little information present that it's impossible to infer whether or not the platforms would be irritating, and even then that's an entirely subjective notion.
3. The lava hazard, which I'm still not entirely sure is a hazard and could just be a background effect, doesn't need to be shown. Besides, it wouldn't be the first time a stage has had multiple hazards. Even Gerudo Valley has Kotake and Koume, as well as the bridge exploding. Norfair in Brawl had lava appear from the bottom, side, and rear of the arena. And again, whether or not that's fun is entirely subjective.
4. You can attribute the model changing to Sakurai having good attention to detail and taking pride in his work. He has an injured arm/shoulder, and he still takes it upon himself to make the game, he's clearly dedicated and hard-working. Lastly, why wouldn't he make changes to the model? Why not just never change models? We could be fighting with 2D Mario and polygonal Link still. The model could just be changed so it looks more appropriate. It's not really up to us to decide what roles deserve a change of design and which don't.
None of these are arguments are very strong. The fact of the matter is we've only seen him on that stage, and he was hinted at being a boss on that stage. I won't dismiss the possibility that he is playable since we have no concrete evidence he won't be, be we certainly have absolutely no evidence he will be, either.