1. The Wuhan institute for virology collected a naturally occurring virus then did gain of function research which subsequently resulted in infected lab workers. There's not a known virus with close enough genome sequence similarity to SARS-Cov19 for this to be plausible. It would be a monumental undertaking to induce >1k mutations in the closest known relative virus. If someone pokes around WIV or a cave in the area and finds a virus with much, much higher but not identical sequence similarity, then that's very strong evidence for gain of function research followed by a leak. In the meantime it seems unlikely that WIV would start doing gain of function research without first publishing about their newly discovered virus.
2. SARS-Cov19 in more or less its current state was naturally occurring in a location that WIV researchers sampled. The virus then escaped while WIV researchers were characterizing it. This requires one to believe that WIV workers, in a biosafety lab, were the first humans to encounter and contract and spread this virus. This is in contrast to the alternate hypothesis that unprotected workers shoveling guano, or maybe a wet market vendor got the virus. I know which possibility I would bet money on.
The point is that we don't have to prove a negative, just weigh the evidence.
The wet market was likely the first superspreader event but the patient zero (from what we know today) had no connection to it.