It feels like you are so very close to a breakthrough. Yes H1N1, MERS, and SARS1 all had fairly definitive epidemiological chains and intermediate hosts identified shortly after outbreak. For MERS they had serological data from camel herds, likewise the intermediate hosts and transmission lines for SARS1 and H1N1 were identified in relatively short time. Shit, they were able to locate the probable index H1N1 case in Mexico back in 2009. There is one recent respiratory virus, however, that does not have a clear zoonotic epidemiological chain. It started in a city home to a chinese lab that had the world's largest field collection of previously undocumented coronaviruses from nature. In fact, that lab was collecting specimens from the sites in SE Asia where the most genetically similar viruses have be located. No epidemiological chain has been discovered that gets that virus from its most likely point of natural origin to Wuhan. There is one scenario that connects the dots of course. Probabilities are hard, I know.