Yes, everyone knows that.
It's a bit different though when you have massive social media platforms that have monopolies over public discourse.
Well, one alternative would be that they have to decouple their front-end and back-end and provide open APIs to their backend (posts/tweets, friends lists, etc.).
Facebook/Twitter can moderate/censor whatever they want on their own front-ends, but they can't remove anything from the backend.
People/companies can build alternative front-end apps that access the fb/twitter graphs and censor in a different way - so that users have an alternative.
(Note that FOSTA/SESTA already imposes a proactive legal obligation to censor within the US, and overrides s230)
No, it's not. If you want corporations to be beholden to the first amendment then pass laws saying that private companies can no longer moderate their platforms, full stop.