As they say the only questions that are stupid are the ones not asked. What you are refering to basically is spontainious combustion. This has never been reported on one of these diesels to my knowledge and is not the case here.

Remember his starter does activate. Just somewhat uncontrollably before he wants it. Also I think it is remaining engaged as long as the engine is running.

This is hard if so as the speed obtained by the starter armature would be higher than designed for. Unless there is an overrun clutch built into a starter.

The most common self starting senario would be to park a 123 diesel on a steep hill with a manual transmission left in gear. If the vaccum system released the shut off on the injection pump. Plus the car started to roll down the hill it could easily self start.