Hi!
About your AU, how did Shadow get his robotic enhancements? His robot eye looks a lot like Metal Sonic's... did he team up with Eggman for a bit just to get himself fixed up? Or did he tear off parts from Badniks and fixed it to his body? Or something else?
Hi!! I think this would have happened early into the 10 years maybe after a year or two!
Tails actually fixed him up! They came across each other by chance when Tails happened to be investigating the same area as Shadow and Rogue. They were ambushed by ancient creatures that had resurfaced, these creatures were kept dormant by the emeralds but with them gone they seek out any chaos energy they can find to consume. Mobians have an innate amount of chaos energy within them so the creatures immediately attacked, spraining Tails’ tails and leaving him unable to escape. Shadow saved his life but lost his arm and eye in the process. The three of them managed to escape back to Tails’ workshop in the Tornado.
Tails felt like he owed Shadow a massive debt, offering to try and construct prosthetics for him and allowing Rogue and Shadow a place to stay in the mean time. In return Rogue found supplies and Shadow kept watch over the workshop, focusing on his recovery as the lack of chaos energy meant he couldn’t just regenerate the damage.
The designs choices were mostly based on robot mobians across the series. I think most of the parts were definitely salvaged, but Tails would have had to rely on Eggman’s designs as foundation for his own designs as he would have had little experience in building those kinds of things!