The "spitting debris" as Nick drove away from his pit slot appeared to be exiting from the lower area of the side pods in the area of the Renaults convoluted exhaust pipes and their exit from the chassis. It seems
consistent with disintegrating engine internals such as valves and associated parts fragmenting and being blown out of the exhaust ports thereby smashing their way along and through the header and exhaust pipes. It would be very interesting to see a picture of the burnt out parts to confirm whether or not this was the case but I somehow doubt that we'll ever get to see one.
The later explosion may well have been that item in the photo, that does indeed look like a pressurised vessel located next to the exhaust system. If indeed it contained nitrogen in liquid form, that's seriously cold and may have expanded at an explosive rate due to the intensity of the heat from the fire around the bottle. That would also explain the apparent "smokey" appearance of the explosion as opposed to a "fiery" appearance, since the sudden concentration of nitrogen in its gaseous form would have "displaced" oxygen in the air, depriving the fire of an essential ingredient (i.e. oxygen), thereby suppressing the fire, even if only momentarily.
I do have a problem with that though since it seems a strange place to put something with essentially cryogenic contents, but yes the explosion appeared to be quite low down toward the floor of the car. the reaction of the marshall, who was struck by exploding debris, appeared to be a response to being struck in the lower leg.
A very interesting case to investigate though. Just the kind of thing I like sticking my nose into (metaphorically speaking, of course
). I'm watching this space and if I come up with anything I will most definitely share it.
Edited due to my not looking at the picture properly!
Hope no-one noticed!