The Sunken Plane (SPOILERS)

[Mo-Nercy]

The Sunken Gelnika may or may not be the same plane that was sent to Rocket Town. Shinra probably has a whole fleet of them. I say this because you can find the sunken ship before you go to space, therefore, it chronologically doesn't work out. It was probably shot down by a Weapon, the game never tells us which one did it but I don't see how anything else could have done it. And about the nose of the plane facing the wrong way...I think you're just reading too much into it. Besides, planes tend to spin out when hit.

The Turks weren't on the plane when hit. They're investigating it. Apparently, the plane had been carrying an experimental weapon of some sort (that's what Reno says when you bump into him) and they had to find out what had happened. I don't think it plays into the story at all. The developers probably just needed a place beside the final Northern Crater area to put the really strong monsters for training purposes. You could say it's the equivalent of FF8's Islands Closest to Hell/Heaven, FF6's Dinosaur Forest, FF9's Vile Island etc.

The Weapon that Shinra killed with the Junon Cannon was definitely Sapphire. Sapphire and Emerald look very different. The former is blue-ish, has a retractable mouth-guard which he uses to shoot laser beams from, he's skinny and swims in a way that's similar to a seal. He also has a tail. The latter is green, shoots lasers from his oversized shoulders, he's very bulky and instead of swimming, tends to chug along the sea floor. He doesn't have a tail either.