A) As long as it's not being regulated by the government, you have no free speech rights on Twitter. Honestly, the government's presence in this story is the only red flag; it could easily be argued that even if the white house asked nicely without any threat of coercion, Twitter's action could constitute a violation of of the first amendment.
With this being said, if all it takes is the presence of a government official merely requesting something to violate the law, Trump will have broken the law in Georgia (re. all he needed was 11K votes). The fact that he's not in jail for it right now suggests that the government CAN be involved in such requests without legal interference automatically taking place.
B) Public health/safety was being risked by the fake news right-wingers crave. The government - as coordinator of pandemic response/efforts - had a duty to intervene.
There's no indication yet that any power was misused. It was certainly used, though.
As long as the banning was solely at the discretion of Twitter, nothing unethical was done.