How common is Gen 2 Head Gasket Failure after 300,000 miles?

The coolant should be at 13 PSI, give or take, so way less pressure than combustion, so should be way less likely to cause a gasket failure than cylinder prssure.

 

I'm guessing that a water pump-to-oil (via timing cover) failure path, if it happens, is likely often assumed to be head gasket and never discovered since the timing cover RTV would likely not be examined in great detail, if at all, and would get replaced when the engine is put back together or never seen when the engine is junked...

 

The timing chain cover, as designed is actually "poor design", except for parts cost/parts count and assembly cost, which is what probably matters the most.
A separate piece for the water pump would have ensured that any possible leak is external and easily detectable (instead of mixing with oil and taking out the engine).

I'm not saying that it's what is happening with my car, just saying that it's optimized for cost at the expense of quality and durability, regardless of how common or rare failures there are.

 

Some more data:
Under the UV light, the inverter coolant already in the car also looks Peptobismol light pink, but cleaner than the coolant (which, I'm ashamed to say, was on my "change ASAP" list when I got the car 50k miles ago...).
An unopened bottle of (Prestone, I think) coolant marketed as Toyota spec is also Peptobismol color under the light.
Brand new oil (synthetic) under the light is white, whereas my oil (with only 325 miles on it) is a very light blue or very light purple.

Interestingly, it looks like with the light I have, dye would not be required, at least not for coolants and oil I have, if searching for an external leak.

 

I believe that they all have the same cover design as my car. I purposely put "poor design" in quotations because it all depends on what the design objectives and constraints were.
If the objective was low cost though lower parts count, lower assembly time, etc. then using a single cover is a success.
If the objective was to focus on minimizing potential coolant-to-oil failure paths and some added cost was allowed by the design constraints (which is what I would have picked, but Toyota obviously didn't), then they failed.
I have no idea if that is the failure mode in my car, but it is a possible failure mode and I believe that a failure there could very often be misdiagnosed as a bad head gasket.