Cancel the service and get Fiber optics.

Are you sure that it’s two cables and not just one that loops back?

Use an Ethernet cable tester/tracer to find where they end up.

Reusing the cable seems like a no brainer, but you may find the cable is already setup like that. Use the tracer.

This seems to be coaxial cable ISP. They are terrible

Powerline IS TERRIBLE. It’s better to use WiFi than Powerline.

Is coaxial running at your place? You could use MoCA adapters in place of the Powerline ones, and its reliability is great, almost as fast as wired Ethernet.

It depends on which devices he had access to. If he was using your computer, he could potentially access your full history, but you would’ve been able to see him do it.

If he was just using your router, it would be highly unlikely he was able to review your history. However, if you’re a high-value target (someone trading with bitcoins or something like that), an hostile party could pretend being from the ISP to try to infiltrate your network.