Those are what you assume. You're cherry picking assumptions just to make it look horrible; I don't see any reason why they aren't perfectly happy there, or are perhaps frozen in time until someone else uses the pool. That video is just a silly joke the fandom made and you're using it to prove a point. The show is a cute kids show; if you're going to claim that it's really some sort of horrible grimdark story, I think the burden of proof is on you.1) See, the difference is that the a fire elemental existed before leaving the Plane of Fire, and continues to live in the fire world after being sent back there. The mirror is not a portal to multiple G3s or anything like that. The Pinkies did not exist before leaving the Pool - something might have existed but not the Pinkies as themselves - and while you could posit that after being exploded by Twilight and returning to the pool as a wisp of magic they reform, I fail to see
how is that any good. Since it's a pool not a world to live in.
And even after all that, you are still left with Twilight putting her friend into a death trap where by design a moment's distraction equals death-or-worse, all because she didn't even consider trying to talk to them first, and Pinkie emerging to tell them in the end that she was the last one left because having friends gave her the strongest will to live. Not these exact words, but unambiguously the same meaning.
And Twilight is Pinkie's friend. I'm sure she knew that Pinkie could pass the test if properly motivated, but the fakes couldn't.
As already established, there are plenty of reasons why he could have become a worse flyer. Sure, it's done for a joke and not explained, but it's not unrealistic enough to be a plot hole. As for the correct thing being to dissolve the team, whuh? I don't even know where you're coming from on that one.4) Bulk in Rainbow Falls can barely move in the air. In Hurricane Fluttershy he was specifically an example that ponies with small wings have no problem flying. Fluttershy is super OOC. Throughout the entire episode the correct thing to do is to dissolve the team because it has no reason to exist. It the worst written episode of Friendship is Magic.
Really? I've seen lots of people suggest the idea that the rest of the mane 5 weren't necessarily friends until Twilight brought them together. I never really considered Pinkie and Rainbow Dash not being that close before the episode a big piece of fanon, it was just the impression I got from the episode.6) Sure they knew each other. But Rainbow didn't like Pinkie and they weren't friends. You are the first proponent of Rainbow and Pinkie not being at least casual friends at the time of the Summer Sun Celebration.
Ok, that's the part you find objectionable. Considering past debates, I'm sure others would disagree, and my apologies if I mistook your argument for past ones8) The objectionable part is where he is unreformed and at full power for an entire episode - at least a whole day, possibly more. Letting him have full power is all kinds of stupid.