This is all hypothetical because it's likely never going to happen. But for the sake of discussion, the radio is just a node on the network. Just like the BCM, or door module or even the tablet running alfaobd. The radio is probably the most advanced node on the network, and probably the only one running a commodity operating system. Unlike a dedicated embedded hardware module like the BCM or other vehicle modules, the firmware/OS of the radio is going to be much more complex, and a much larger file.
It's probably not possible to load/update the uconnect OS via the can bus. I doubt the bootloader supports booting from a low speed network like CAN. Even if it did, it would take forever to transfer an OS image via CAN. So i doubt something like Alfa could ever touch the apps or OS on the radio. I would bet that the OS Image transfer is only supported via either USB or WiFi or cell network. Which appears to be the case since that's how updates are done.
On my old truck with the UC4, I once took apart the update package file on a computer after downloading it from the uconnect site. It was simply an OS image file. The UC4 used the QNX operating system which is a linux variant. Most of the UI apps were flash applications. There were some really curious files in the image package file, including several mp4 videos of crash tests. No idea why that would be included in the radio's memory. The image package however was signed, so any attempt at modifying or adding to it would cause it to be rejected by the bootloader. UC5 is probably a similar signing method, just android instead of QNX. Nobody ever cracked the signing of the UC4 system. That always surprised me. Perhaps because it was a more obscure OS. Perhaps because there's not as many tinkerer/hacker types in the auto enthusiast community. Perhaps because FCA doesn't sell cars into the countries where the best hackers are.