For the most part, those "viewpoints" are verifiable falsehoods, conspiracy theories, and other types of disinformation and misinformation. For instance, the recent migrations to Parler are chiefly people who want to deny the legitimate US presidential election result and spread false claims of widespread vote fraud.
Using the term "viewpoint" like you did is misleading, since it has the effect of making literal lies sound like legitimate differences of opinion.
> 3. This marginalisation leads to increasing extremism; going from the likes of “immigrants bad” we saw in 2016 to disputing and attempting to overturn democracy.
Eh, not so much. A lot of the recent "increasing extremism" (for instance QAnon and Boogaloo Bois), occurred on mainstream social networks like Facebook and Twitter. While pushing crazies and extremists to places like Gab and Parler probably radicalizes many of them further, it also isolates them from the wider community, which helps reduce their ability to recruit new people into their lunacy.