James
I wouldn't think you need an explosive to set off Rocket fuel like you need one for ANFO or C4 etc etc.
I'm sure their were plenty of volatile things in the factory - which proved correct !!!
You've always got static electricity as an accidental source ?
I would say the site relates to your " 3) there was originally no risk and this is a plant subverted from its original task".
Re the Berm, on other "bigger" pictures of the Iran site it does look like their is a wall or something around the whole area but quite some way out.
Even a small, angled earth berm around a building will direct the majority of the shock wave upwards. As long as whatever it is, especially earth, impede the shock wave, it will stay intact. ie the Old Bomb Explosive bunkers of yesteryear that had the earth berms around them.