You're making assumptions. I've used GNOME since 2008, with extensive use of KDE and Cinnamon too in later years. I've used other DEs such as XFCE for niche applications. KDE - for me - has always been buggy despite being the most feature rich. This is just my experience. You don't have to trust me.
Edit: Currently daily driving COSMIC. It was a little rough in the Alpha stages but it's now very stable for me.
It's hard enough getting legit software in general to work on Linux. Even if a virus was written for Ubuntu, it is likely not going to run on Fedora, or Arch, or even downstream/upstream versions of Ubuntu.
Edit: Although thinking about it, Linux terminal commands are pretty universal, so if you manage to execute a script or terminal command as root or sudo then I guess it could apply to multiple distros.
Right-click on the applet (where you want to middle-click), select 'Applet preferences' -> 'Configure'. Change the 'Middle click action'.
Cinnamon I've found to be buggy too. Panel and wallpaper glitches, black screens on wakeup, to name a few.