AFAIK there is no way to play he multiplayer locally, Doom Eternal has an online fix via MS Store tha you can download from online-fix.me. As for the multiplayer features, 2016 is more like the multiplayer mode in the OG Doom, Eternal is a 2v1 2 Demons versus 1 Slayer. They are very different so you cant really compare them.

Fitgirls repacks (and others aswell) should have an option to limit cpu usage (or ram usage, cant remember) which should stop this if you are running into issues or want to use you pc while installing. Having a component pinned at 100% is fine, they are designed in a way that if they become unstable/overheat to clock down and run slower to stop any damage from occuring. Install times vary due to game size, since RDR2 is a pretty big game its going to take a while to install. If you want faster install times you could download a bigger repack or download the release the repack is based on, they will install faster because they need to decompress less or not at all but download times will be longer.