Guests who don't really know what they are talking about and like stoking wild conspiracy theories. Maybe those guests truly believe what they are peddling, but it is certainly not true that the claim has been "proven". Rather, the best current evidence available suggests it was natural and not man made. In any event, the virus exists and the true danger of it is not yet fully known. The problem for the government is that it has the potential to be really bad. So the prudent thing to do is to take steps to guard against that. It may later prove to be less serious than that potential, but until we know that it's better to be safe.
Wrong on both counts. The Constitution's right of free assembly is not absolute. None of the rights in the Constitution are absolute. The Supreme Court has held that the right of assembly can be regulated by the government when there is a compelling state interest, like protecting the health and safety of the public. Quarantine laws, so long as they are no broader than is needed to deal with the public health risk, are indeed constitutional. The fact that the courts themselves are closing, often on the order of the state Supreme Courts themselves, ought to tell you that the courts are not going to object to most of these efforts to contain the virus. There may prove to be some officials who do overstep their authority in their zeal to protect the public, but to say that none of what the government is doing to combat the virus is illegal is simply wrong.

