Possible problems with the resolution criteria:

• I don’t like the fact that "PHEIC" and "pandemic" are used interchangeably. The resolution criteria are way less stringent than the title of the question and the accompanying background information suggests. This is somewhat misleading
• “A new plant disease outbreak is announced” seems to set the resolution bar dramatically lower for plants than for humans and animals. I’m also quite confident that there is a lot of bioengineering already happening with plants, so the probability of an Epiphytotic event might dwarf the chances of a pandemic or panzootic one
• In the event of a high-consequence outbreak, absent an immediate clear indication that the biological entity has been bioengineered (e.g., an admission of responsibility or credible evidence of a specific incident), it could take months or years to determine the pathogen's origins conclusively.
• “High consequence events which occur due to biological entities that already exist in their current form outside of a laboratory setting will not count.” I find this criterion somewhat problematic, but that might depend on my ignorance of the subject. I believe at least a few currently well-known pathogens have pandemic potential and are both found in nature and studied in labs. It might take some of them just a handful of mutations to completely alter their transmissibility or mortality rates, and cause a pandemic. I would expect the criterion to be satisfied if the event is triggered by the engineering of a new previously non-existing variant even if it differs by only a single mutation.

This question puts way too many irons in the fire: it asks for a single probability estimate for a wide range of possible scenarios over a decently long time frame.