1) "Es" refers to whatever happened in the part you left you out. For example, if Gerd is saying something, "es" would refer to what he said, and "kam es von Gerd" means "Gerd said that".
2) Just as in English, you can use participles (both present active and past passive) to form a phrase that acts like an adjective or an adverb. In this case, it acts like an adverb: "wie vom Donnerschlag getroffen" describes an emotional state ("totally shocked"). You can split off the participle-phrase and turn it into a phrase of its own: Er war wie vom Donnerschlag getroffen und blieb stehen.
3) It's a circumposition: A preposition is a word before an object, a postposition is a word after an object ("ein Jahr lang"), and a circumposition consists of two words, one before and one after. So "von X aus", "zu X hin", "um X herum" usw. "Von X aus" is stronger than "von X", and emphasizes that the action takes place coming from X.