On the first point, I have already posted instructions for downloading the UEFI boot installer:
The Instructions for downloading the BIOS boot version are the same apart from step 3 where you would select “Native PC (BIOS/CSM)” instead of “Native PC (UEFI)”.
After step 3, you will be provided with a page that looks like (e.g. for the UEFI boot version):
This provides links to both the download image and to the install instructions - make sure that you select “Native PC” and then either (“UEFI installer image” or “BIOS installer image”).
For convienience, the UEFI boot instructions can be found at How to install DietPi (UEFI) - DietPi.com Docs and the BIOS boot instructions can be found at How to install DietPi (BIOS) - DietPi.com Docs
Other than the fact that there are two different images, one for BIOS boot and one for UEFI boot, the install process is very similar to that of ROCK (download image, write it to USB stick using Balena etcher or rufus, Connect monitor and keyboard to NUC/PC, boot NUC/PC from USB stick to install).
On the second point, I have never had any issues with ARC using port forwarding - but I keep my network very simple (ONT, and ASUS router, unmanaged switches) and my ISP does not use CG-NAT. I do not use uPnP. I have manually configured a port forwarding rule.
It has to be said, however, that, on the same NUC (when I originally set it up), my ROCK install also worked without issue.
In fact, going from ROCK to Dietpi did not even involve a change to my router setup because I had a DHCP reservation for the NUC MAC address and thus the DietPi installation used the same ip address as the prior ROCK installation.