I would like to add another factor in your favor for this hypothesis. Naturally occurring viruses/bacteria tend to evolve to spread most effectively in conditions similar to how animals live: outside. COVID-19, conversely, seems to fair poorly outside and spreads most rapidly in indoor conditions not unlike one would expect in a human run laboratory environment.
There's a lot of bats, caves, and viruses. Virologists find new viruses all the time, why would you expect that they have surveyed enough that the fact that they haven't found a particular one is evidence it's not present?
A virus with a novel mutation favoring humans would be successful infecting humans, period. I can't imagine any respiratory virus that wouldn't spread more effectively indoors. This "factor" is not particularly compelling (to me).
