The software used was a commercial tool made upon Guacamole (Apache) called Inuvika. It's pretty awful, but having Linux and Windows apps on the same virtual desktop is quite cool. I don't know how much of that functionality comes from Guacamole or from the Inuvika addons.
Inuvika/Guacamole also support plain RDP, but we didn't use this, just the html5 client (browser)
If you want to see what open-source can do then look at Guacamole and go from there.
If its alright for you to answer,
1. What would be the best/cross platform way to proceed?We now have separate agents for windows, mac which causes maintenance hell
2. Is chrome remote's way of streaming desktop images as video better than images + diff.
3. Is there any open source mirror driver kind of thing in linux?