It’s probably an issue with a public IP or port settings. I really hope that in the future the game will be released on Steam with proper, fully functional multiplayer for everyone, because after a year and a half my friends and I came back to 0 A.D. and once again found ourselves trying to figure out how to join the lobby — using VPNs, Radmin, and other tools just to play normally, which doesn’t really help.

I didn't try it but take your word. My question would be thusly: Why? Why on earth is the windows installer so extremely user-unfriendly when different versions aren't technically a problem? Could this be changed for future versions?

Well, both, I suppose.  

Actually for me it is [c] (lower case).

Maybe this helps: How to: Using two different versions of 0AD on the same windows machine: