Screen sharing under gnome and KDE just works. Even in sway that's solved for quite a while now. I would call that widely adopted. I haven't encountered the other two as problems yet. Global shortcuts work in my sway session. I would expect nothing else from gnome and KDE. Window position restoration is also a standard feature in sway AFAIK. Not sure if gnome or KDE do it.
Yes sure. But at some point you have to willingly accept that "everyone" out there is lying except your propaganda outlet and disconnect from reality. That is so obviously BS to me that it's hard to imagine to fall into that trap. Even though I witnessed it first hand with friends and family.
One of the design goals is that they don't have a user database, so governments etc can't knock down their door demanding anything.
By using phone numbers your "contacts" are not on their servers but local on your phone.
Not sure if you understand threat models.