Why is there a carnival zone on Angel Island of all places?? Did Robotnik have time to just build one? That always bugged me, the rest of the zones made relative sense in one way or another (despite, you know, radically different climates being right next to each other on a floating island).
I'd guess it was one of the most recent things that the Echidna civilization built before they disappeared. Knuckles maintained it because he was all alone on the island; the carnival and its maintenance were his only sources of amusement.
It's the surrealism of the Mega Drive trilogy (+ Sonic the Hedgehog CD). Spring Yard didn't follow a level theme (It wasn't really a city level, it wasn't a casino level, and it wasn't a mountain level...), Collision Chaos is a pinball table... "On drugs", and Carnival Night looks out of place in Angel Island. Actually, Sonic 3 has the most "normal" levels in the trilogy, so Carnival Night looks even more out of place.
Spring Yard Zone appears to be explained in that Sonic The Hedgehog's zones are arranged incorrectly - the original game's level select screen has them in a different arrangement of Green Hill > Labyrinth > Marble > Starlight > Spring Yard > Scrap Brain, which seems to show the stages becoming less rural and more industrialised as they progress, matching the theme of getting closer to Robotnik's main base. This was better enacted in Sonic 2, minus the placement of Chemical Plant Zone.
Alternatively, Carnival Night is Eggman's primary base on the island, while Launch Base is just the subset dedicated to getting the Death Egg airborne again. The Flying Battery's gotta refuel somewhere, and the level order was originally going to have Flying Battery immediately after Carnival Night, anyway...
Why do the spitting statues in marble garden appear human?
Without knowing who built them, it's impossible to say. Humans could have built them. Or, just as humans make animal statues, anthropomorphic animals could build human statues. They might be religious.
It was built by Echidnas who have probably never heard of humans.
Nah, it was most likely built by humans, along with the Sandopolis pyramids, whose walls feature carvings with human figures. It was never stated that the Knuckles clan were the only beings in Angel Island. There might have been human civilizations that lived there in the past but became extinct, or alternatively, Marble Garden and Sandopolis might have been built before Angel Island came to be.
How did Knuckles knock Sonic out of Super Sonic mode? He even did so hard enough to knock the Chaos Emeralds out of Sonic. Isn't Super Sonic supposed to be invincible, at least for the early games?
Better question. Why does he hide the stolen emeralds in poorly hidden giant floating minigame rings?
Maybe being guardian of the Master Emerald just let's him do h4x shit like that? For all we know, canceling a Chaos Emerald-induced Super Mode is part that power set, like sensing Emeralds - not like he's ever had occasion to do it again.
Actually, in one of the cut-scenes in Sonic and Knuckles, Knuckles knocks a giant boulder into Sonic, which can revert him back to normal from HYPER. Since he's an NPC in Sonic's story, he's invincible in cut-scenes, but a joke in actual battle.
When Knuckles punched him out of the super form you start the level with no Rings. Maybe the transformation was just about to end and Knuckles had an extremely lucky shot.
It's a canonical fact that the Master Emerald can nullify the Chaos Emeralds. Maybe Knuckles used its power to cancel out Sonic's Super form? He might previously have "charged" himself with the Master Emerald's energy in order to nullify Super Sonic, or maybe he simply had the Master Emerald with him and used it (remember, SA 2 heavily implied that he can shrink the Master Emerald and carry it around).
Sonic 3 And Knuckles had Knuckles going through Sonic's Angel Island Act 1. Since his story takes place right after Sonic's, wouldn't it still be recovering from the fire?
Well, in canon, Knuckles' gameplay never happened in Sonic 3...I don't know.
Why did Angel Island fall from the sky in Sonic 3 anyway? The Master Emerald was still in Hidden Palace at that time.
Eggman's ship crashed into it, and the sudden force propelled it down.
Yeah, S3&K's backstory stated that the Death Egg knocked the island into the sea, and after that it's actively pushing it downwards. As the game shows, the Master Emerald is capable of lifting Angel Island OR the Death Egg, but not BOTH at the same time.
Why did they show the Death Egg getting destroyed in the Sonic 3 ending? They already knew that that story would be continued in Sonic & Knuckles. Granted, S3&K alters that cutscene to show the Death Egg falling rather than exploding, removing any continuity problems. But still, why wasn't the Sonic 3 version like that? For people that play the game without Lock-on, the Death Egg's presence in S&K will make no sense.
To make Sonic 3 seem more like a complete game for those that were unaware of S+K or couldn't buy it. And to be fair, the continuity of the games was extremely loose at the time, meaning it was hardly that big a deal.