I had a longer response but my browser crashed so I'll just try to summarize my thoughts.
Our disagreement basically comes down to how we assign priors. I'm not really sure how productive a debate on priors can be because it's not like there are a bunch of previous similar scenarios that we can study in detail.

On P(WIV leak):

Wikipedia[0] lists 50 lab leaks (or more precisely, failure of lab biosecurity measures) in the past century. The actual number is probably much higher because people and institutions like to hide their mistakes. So the base probability of a lab leak happening from a lab that studies pathogens is only moderately low to begin with.

But P(WIV leak) is probably significantly higher than average here, because WIV had worse than average safety procedure.

- According to leaked cables from 2018, american officials & scientists were already concerned about unsafe practices & lack of properly trained personnel at WIV[1]

- WIV researchers collected unknown viruses from bat caves with only hazmat suits + surgical mask as PPE, and handled vials of viral vector specimens without even masks[2]. If they demonstrated poor attention to safety outside the lab, it is likely that they disregarded it within the lab as well

- I recall reading something similar involving Shi Zhengli but I can't find the link now

- Shi Zhengli admitted (!) that bat coronavirus research was performed under BSL-2 and BSL-3 conditions, both of which are significantly more lax than BSL-4[3]

On P(last bullet):

It's a pretty specific scenario, so this probability is very low. It's specific in a way that's meaningful and relevant though, it's not like the specificity came from extraneous things like "and the virus' genome is between 29 and 30 kb" or "and the outbreak occured in a city whose name starts with W".

On P(last bullet | WIV leak):

This probability is essentially a function of the infectiousness of the viruses in the WIV.

> it's very unlikely for a mere exposure to turn into a pandemic. Exposures to natural potentially pandemic viruses happen every day, most don't become a pandemic.

Strongly disagree. The viruses in WIV were deliberately altered, via either natural selection in host cells or direct genetic manipulation, to be significantly more infectious in human cells than a random virus found in the wild would be. It is unlikely for human exposure to a random new virus to lead to a pandemic. That is not the case for human exposure to a new virus that has been altered to be much more competent at infecting humans. Especially considering that these viruses we're talking about are coronaviruses, which are already significantly more infectious than the average virus. SARS2 also has features that are the sort of features we might expect from a virus in the WIV, such as the furin cleavage site and competence at infecting cells through ACE2. WIV research definitely focused on the latter and possibly on the former, so this attribute of the outbreak also increases P(last bullet | WIV leak).

There is also one known precedent for (some pandemic outbreak | some lab leak) which is the 1977 H1N1 flu epidemic. It isn't known with certainty to be from a lab but there is a general consensus that it probably is.[4]

Sorry for not providing numerical estimates for any of these. That would feel super handwavy and they would be, like you admit, entirely unconvincing. Besides, any motivated person can adjust his priors until Bayes spits out the desired outcome. In any case, I feel that the WIV leak theory is simply a more parsimonious explanation of the particular attributes of this pandemic than the zoonotic origin theory. However I doubt there will ever be a "smoking gun" for either theory.

[0] https://en.wikipedia.org/wiki/List_of_laboratory_biosecurity...

[1] https://www.washingtonpost.com/opinions/2020/04/14/state-dep...

[2] https://www.youtube.com/watch?v=ovnUyTRMERI

[3] https://web.archive.org/web/20201206204844/https://www.scien...

[4] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4542197/