ARC protects you from most common kind of Actions incident as seen over last month: GitHub's runner pool running short. Nothing CI-based can protect from an outage in the Actions service itself.
It should be possible to wire another event dispatcher to ARC instead of relying on actions webhooks. That’s the only part that is a true dependency, it should be possible to basically decouple ARC
Ideally yes. As a backup path I've considered Forgejo with a pull mirror and forgejo-runner on Kubernetes but haven't pursued it.