Nah, anything will work fine.
Just a quick question, are you sure it’s the cpu that died? Those ivy bridge (?) chips really seem to last. I’d be surprised if it was the cpu and not the motherboard or power supply of something.
You have one of the nicer fourth gen chips. It would probably be worth it to take it to a computer shop or something and have them try to boot it with a good board.
If it’s still kicking, those motherboards are cheap as heck. The ddr3 is cheap too.
No reason not to keep it around to run a file server/seedbox/Jellyfin server/whatever.
Yes you can!
As you said, it’s got everything to do with routing and you don’t know how to do that yet.
Now’s a great time to learn!
If you’re on a time crunch, go ahead and use network namespaces under network manager to set up something like what you want as another user suggested.
If you have time to learn about the firewall and routing table rules, put on your wire rim sunglasses, pop a jungle cd in and crack open Linux Firewalls or some such book for nerds.