Hello,
First there are no definitive answers, we are talking Siberia circa 1908.
Fur traders and villagers were the only witnesses, they aren't exactly professionals when it comes to trajectories and or good for precise accounts - most thought it was something involving the gods.
Just speculations on your questions :
Preceding atmospheric activity -Apparently after the initial explosion there were several other "gunshots" which were fragments/parts of the main (asteroid/comet?) - perhaps the effects that occured prior to the main explosion were also fragments of the asteroid/comet but ahead of the biggest chuck - leans toward thinking it was a comet.
Magnetic storm - no idea. Maybe the asteroid/comet? had been extremely "magnetized" from various trips around the solar system/sun
and somehow this "magnetized" energy was suddenly released or perhaps earthquakes exposed/disturbed a highly magnetized area uder the earths crust.
I quote again on the debris -The search for meteorite fragments now seemed fruitless, because computation of the explosion parameters, together with the nature of the geophysical records, revealed that the density of energy at the moment of the explosion was extremely high.
Personally I think it was a semi-loosely bound comet (magnetized from several trips around the sun/stars?) whose fragments both preceded
and followed the main event.
I have some real reservations about your initial ground based hypothesis, for one - it doesn't seem to match any of the current/previous described observations.
see ya, VINCE