I think the answer to a) depends on whether Twitter wants to continue to have a private monopoly on public discourse. If Twitter wants to maintain that monopol

I think the answer to a) depends on whether Twitter wants to continue to have a private monopoly on public discourse. If Twitter wants to maintain that monopoly and their walled garden, then no, they should not be able to arbitrarily ban users. I would like to see online public discourse declared to be part of the social infrastructure and be treated like a utility. Alternately, if Twitter is willing to move to a federated-type model where Twitter users can follow Parler users and vice-versa, I think they should be able to continue to ban people. That neatly resolves this conundrum to me. Twitter loses its absolutely staggering inertia from the size of their userbase. The data becomes open, and social media companies become focused on providing value via exposing you to this data, rather than just by building a moat with it to keep out competitors.