I think one of the motivations for having separate modes like this, with (some) separate registers for each, is to reduce the time taken to switch contexts between modes. If they didn’t have separate registers, the data in the user mode registers would have to be saved somewhere when making a switch into kernel mode, and then copied back again when switching back to user mode.

A few years ago, I used dosdude’s “patcher” to install 10.13 on a pretty old MacBook Pro that couldn’t run it. It’s been working really well. I think “OpenCore” might the more modern version of this. Worth a look?

Apps like Halide can give RAW files, which I think gives access to data much closer to what the sensor actually recorded.