How can I use Proton VPN on multiple devices?

Yes, but you may need to try specific servers optimized for bypassing restrictions.

Open the app, go to settings, and toggle the split tunneling option to select which apps you want to tunnel through the VPN. The steps to enable split tunneling in Android and Windows is same. However in Linux, you have to open the configuration menu with protonvpn configure, then select Split Tunneling with '6' and confirm with 'y.'

This can occur if the VPN isn’t configured properly. Ensure that the VPN’s DNS settings are activated in the app.

Proton VPN provides a money-back guarantee for 30 days. Unless you purchased with cash or a bank transfer, you can receive a complete refund if you cancel within this time frame. Depending on how long your plan was in effect and how long you utilised the service before cancelling, you might also qualify for a prorated refund.
Go to your account settings on the Proton website, find the subscription section, and follow the cancellation prompts.

On all of its servers, Proton VPN uses full-disk encryption (FDE) by default; hence, no action is necessary to enable it. Each exit server's software, configurations, and server certificates are protected by FDE. This prevents attackers from accessing the server even if they manage to physically enter.