From what I've saw, Valve servers are offline so no official MM. You can play with bots, and play on community servers, but you have to connect via "connect" command in console, because community server browser is broken.
Skeletonek
It's not removed. You can still access latest CS:GO version via beta tab in game properties. Is it inconvenient? Yes. Is it shit that achievements for CSGO were removed? Yes. But you can still play it, and play on community servers.
I really don't know what you are talking about. I'm using it on Linux and it runs great. Comparing Signal to Discord, as both are electron apps, Discord is pure shit, Signal works really well. It takes a couple seconds to load but it's still faster than Discord for me.
I would prefer if it would use other cross-platform framework like Flutter or Qt, but you can't have everything. It's better to have electron app, than don't havy any.
I wouldn't use Proton to play this game as it had more problems on my machine than native Linux version. Try using Steam Linux Runtime in Steam's compatibility tab.
It is, I have it set up on my laptop. It's a bit finicky in how it works and it's not easy to setup, but it is possible.