Here is a loose theory about the lens (window) blowing out.
The PM6 in question is an early model with focusing capability. The LA (lamp assembly) is separate from the reflector unlike in the second iteration of the PM6 which is non-focusing. The LA screws into the battery tube. This results in two separated atmospheres, one existing in the head and the other existing in the tube.
The second path of least resistance for gas/flames/pressure would be the potting surrounding the lamp. The first path would apparently be the tail switch. Before the first explosion there was venting and gray haze coming out of the tailcap.
It could be that there was also some unseen fiery out-gassing from the anode end of the first battery that burnt and pushed a small hole through the potting. Or...it could be that the first explosion blew a small hole through the potting. Also, during the second explosion a jet flame was observed coming out of the tailcap.
I'm thinking that one of these events blew or burnt a hole through the potting surrounding the lamp. That would cause the head chamber and the battery tube chamber to share the same atmosphere. When the second explosion took place the lens received quick, high pressure and blew out in shards carrying toxic material.
If we look closely at the glass envelope we can see a black wisp on it. This may be evidence of the defeated potting and a small yet powerful jet-blast flame having traveled close to it.