An RPI running android still won't be sitting on the right bus to interface with the audio system, you'd need to get something that would plug into the optical bus system and interface with the i-bus. Torque on Android isn't going to do it, I could put you in touch with some embedded developers who could do this for you but it will cost you way more than just buying a CD player and installing it. There's a time to be pragmatic over this and a time to try and be clever. There's many things on the GM900, 9000 and 9-5 you can only do with a Tech-2 tool and although there are probably ways around it i'd say it's not worth bothering with.
Anyhow, not sure why you mention Microsoft and how your computer can do anything that mine can, I've been using Linux pretty much exclusively for well over 15 years now and use it daily in my current day job. Saying that, I do have a Windows VM which is exclusively used for testing and running the Saab workshop software.
It is worth pointing out for the audio system on the 9-3 you could get an auxiliary input which fits into the centre storage cubby, it's about £20 but enabling it requires Tech-2 so I don't know if it's an option for you and there is also an aftermarket unit that will give you an input into the audio system but it's about £500!