This really doesn’t answer all your needs but the Microsoft Office 365 office and productivity apps are available as web apps, and work very well on Linux.
Agree. That's how we are using it. However teams webclient can't do some of the things like desktop sharing and conferences like we could do in hipchat.
linux has a native desktop sharing, and if you secure things properly SShing to eachothers Boxes over an inhouse VPN.