>although I'm not sure why they have there own OS.
I think this makes sense. Their core bussiness is producing an integrated Linux hardware/software package. When a compatability issue comes up, they probably don't want to have to wait on a third party to resolve it; especially when they can still leverage most of the work that said third party does (and; I presume, contribute back their work).
Anyone can hold any status in Ubuntu regardless of who they work for. This is explicitly stated in Ubuntu's code of conduct which doubles as a sort of constitution. They could be Ubuntu developers themselves; then they wouldn't have to "wait on a third party" to resolve anything.
Yes but anything that goes into Ubuntu needs to work on all systems, not just System 76 devices. This would like reduce the realistic rate of change, even if they were (co-)maintainers of all the relevant packages.
Not necessary to upstream packages to Ubuntu Repositories when they could set their own repositories (which I assume it's how they mantain their distro)
It's a matter of branding. Which is great too as long as they keep up to date with security updates and such.
I don't think Canonical would look kindly on someone shipping 'Ubuntu' with 3rd-party modifications to critical system-level packages, at least without participating in the Ubuntu Desktop certification program.
I think this makes sense. Their core bussiness is producing an integrated Linux hardware/software package. When a compatability issue comes up, they probably don't want to have to wait on a third party to resolve it; especially when they can still leverage most of the work that said third party does (and; I presume, contribute back their work).