You can definitely run an HTTP server behind WireGuard. WireGuard just adds a network interface that your server can listen on (e.g. your server would listen on a private address like 10.0.0.1).
OP's example was a great one. For example, let's say you visit your friends and you want to watch some content together that is accessible on your server. Using this approach you can open access to that without setting up Wireguard on friend's smart TV.
And Wireguard does use quite a bit of CPU if you are using a lot of network bandwidth. Small servers don't have that much compute power, so utilizing the port knocking somewhat removes that issue.
WireGuard has apps for most devices (macOS, iOS, Android, Windows). For smart TVs it's a bit of a mixed bag. Some of them do support VPN clients, and I know Tailscale works on the Apple TV now (Tailscale uses WireGuard under the hood).
If you're using the Ruroco client to proxy requests to the server, then you could do the same with WireGuard. You could have HAProxy (or something similar) proxy requests from your local network to the WireGuard interface.
No, the example is - you go to friends, join their Wifi, have the same external IP, send the knock from your phone/laptop to open 443 to that IP, and then you can connect from TV or their computers.
Port knocking only handles authentication (it's basically a crude password). It doesn't ensure the integrity (tamper prevention) or privacy of your connection. You would need to set up SSL certificates to handle that. You also need to get the TV to accept those certificates, which would require either a public DNS record (which exposes your server's IP via the certificate transparency log to any client that can issue a DNS request), or you would need to modify your friend's router's DNS resolver (which is even more complicated than installing a VPN, and assumes the TV uses the router's DNS server). A TV isn't going to have an /etc/hosts file that you can use to point a domain at your server's IP address.
So instead I would go to my friend's house, connect to their Wifi, and accept incoming connections from computers and TVs over the local area network and forward them through WireGuard. The TV would connect to my device via plain HTTP (which is fine since it's all happening locally), and then my device would be responsible for securely connecting to the server via WireGuard. This also has the benefit of implicitly revoking access as soon as I leave their house with my device.
My NUC easily pegs the network. I'm not sure you're right about this. Either way: you can just use WireGuard as a control channel, the same way this software does.
And Wireguard does use quite a bit of CPU if you are using a lot of network bandwidth. Small servers don't have that much compute power, so utilizing the port knocking somewhat removes that issue.