I'm mostly agree with everything.
The only thing I could say (100% pure speculation, this is not real or fake) is that the Sonic game for the Saturn was going to be real but Naka dropped the idea and later he returned to work for Adventure after finishing Nights.
The question is why he dropped that? Well...it could be anything. The most probable thing could be that they didn't find a way to move Sonic fast enough and controllable in 3D for the Saturn.
But one thing for sure is that "what they have worked, they have utilized in someway in another games"...Nobody thought about those correct 3D models for Sonic & Co. that they appeared on Sonic 3D, Sonic R and Sonic Jam' Sonic World? or the appearance of Metal Knuckles, Tails Doll, the Egg Robo, Amy and Metal Sonic on Sonic R? When or where Traveller's Tales got that idea?
Summarize this questions with the revelation of Knuckles Chaotix as an intended Saturn game for 95 and you might have more interrogatives.
And also, from the "Mastermind Trio" (Naka, Ohshima and Yasuhara) , Yasuhara was the only original member of Sonic Team who he stayed in USA and didn't work in Nights neither Burning Rangers or even both Sonic Adventure games.
What did he do? Well....he supervised/designed for both Sonic 3D Blast and Sonic R. He even helped the team for the Boss Engine for Xtreme.Could it be that this guy handle all the beta working material already done and latter offered those works for both "supervised" games? It's a really strange situation that even became more stranger when you find this promotional material for those games:
This was supposedly the protobox art for Sonic R and it was changed because Metal Sonic appeared in the cover and, in the game, he is a Secret character. Seems plausible,does it?...... The art guys from TT were really talented replicated exactly the style of Sonic Team for that art...
Their coincidences are uncanny.....