I removed the tensioner and found this chain pad...
The blob that was on the gear (although I guess it is more correctly called a sprocket) appears to be the right size to be the missing piece of the pad. It is so deformed that there is no surface that lines up with the rest of the pad.
Since this is the only anomalie that I have found so far it would seem to be the source of the broken valves... but I am having difficulty visualising how a double row chain with about 75% contact on the timing sprocket could skip because one link is not making contact.
Should not have doubted Homeoboxter.
I would guess that the engine has at more than 100K miles on it. On other engines that I have disassembled, with less than 100K miles the pads were worn but not broken. In my opinion if the pads have not been updated at 100K miles it is definitely time to do it. This question is not discussed enough on forums that deal with the M96 Boxster and 996 engines.