Can a VPN service be used like a reverse VPN allowing inbound traffic for serving a website?

What you want is called 'port forwarding' (or similar) by commercial VPN providers. What they do is put rules in their firewall that forward packets arriving on your assigned port to your internal VPN IP address.

You need to configure the Cloudflare Proxy to send connections to the external ip address and your assigned port on the VPN server you are connected to. For example if you are assigned port 14386 and connect to a VPN server with an external IP address of 192.0.2.22 you would configure the proxy to connect to

192.0.2.22:14386

Depending on the VPN service, you may get different external IP addresses whenever you connect to the VPN. You will need to detect this change and update the proxy information as required.

You will also likely get different internal IP addresses. You may need to take this into consideration in your firewall and service configurations.