Isochron Scepter creates a copy of the imprinted card, which is then cast as part of the ability. Like all spells, the copy created uses the stack and can be the target of counterspells. Stifle will stop the ability from creating the copy. Counterspell will stop the copy.

Does Isochron Scepter add to storm?

Isochron Scepter has you cast the copied card which will indeed cause storm to trigger.

Does Isochron Scepter trigger Guttersnipe?

Yes, it does. Isochron Scepter (unlike some similar cards) says you copy the exiled spell and then actually cast it, so anything that cares about casting (like Guttersnipe) will still trigger.