Is it possible to use Opera’s built-in VPN as a general VPN for an entire laptop?
No. Opera VPN is not a true VPN. It’s just a HTTPS proxy marketted as “VPN”. It’s no different than those browser addons which call themselves “VPN”.
If you want to use Opera “VPN” web proxy for applications outside of Opera (which can use HTTPS proxy; e.g. for Firefox; note: must be manually configured), you can use below proxy wrapper application, but it can only use one location per application instance. Opera doesn’t need to be running to use it.
Unfortunately no, it’s only intended to tunnel the network-traffic from the browser.
Opera VPN works only inside Opera.