I'd like to see where the pinpoint holes were in the radiator.
BTW, my van had a relatively exposed radiator ~ that was due to the fact the engine compartment was larger than that in a car. You could climb underneath and look right up at the radiator without any obstructions. My cars have always been more enclosed. It may be easy to hit the radiator in this particular BMW.
If you have a hot car suddenly spewing hot coolant as a fine spray (with teeny tiny particles with enormous areas compared to their mass) and the catalytic converter is hot as can be THEN that may have done the job.
I don't believe the lightbulb deal myself.
Upon further thought, if the radiator truly has a punched hole, there are possibilities. If, as you proposed, she remotely started the vehicle & let it warm up before she got in, the cooling system would be somewhat pressurized. It is also possible that she started to back out and decided she was too close to hubby's car and pulled up for another run, impaling the radiator and and also breaking the lens of the headlight. That would allow the antifreeze mist to contact the hot bulb and ignite.
Not sure if the blast did her in, but that seems to be the case if the observation of her being found upright in the driver's seat is true. If she were conscious, there would be evidence of trying to escape.