1) https://assault.cubers.net/docs/commandline.html Try to set "--mastertype=0", maybe it will help. Otherwise see if you can ping or do other general connection tests to assault.cubers.net:28760. Maybe you can't resolve the domain but you can reach the IP and configure that as the masterserver using the -m flag. Next try the forum at forum.cubers.net as this place will be slow to respond.
2) Not as much as before. Still enough to give you a good taste of it. I say you've missed the golden age.
3) Xonotic and Warsow are nice but I think they're relatively dead now too. Try looking into shooters for Linux because these will generally be cross-platform and low-resource.