Conflict with other webservices running on Pi (Ports 80 and 443)

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Sorry for the short answer but its such a lengthy story that i missed out on the 8080 bit.
You could setup Pi with two IP addresses on one and the same network interface.
And have lighttpd listen only on the one IP address and nginx on the other IP address.
So Pi-Hole can go back to using port 80 for the admin page and displaying empty ads as it was designed for.

So for example:

10.0.0.10:80  --> lighttpd (for Pi-Hole)
10.0.0.20:80  --> nginx (your other web apps via HTTP)
10.0.0.20:443 --> nginx (your other web apps via HTTPS)