Lol, most of the sentences i've seen can be quite sinister. My first thought was some crazy hillbilly and his brood slipping through a twilight clad window of a remote cabin in the fjord hills, then i remembered mosquitoes and ants, spiders and other such creepy crawlies, and decided some loony and his ilk aren't so bad lol
"inn" is an adverb that describes how a verb is done. "...come in...." "inn i" is a preposition "into" and it would not make sense in this sentence as there is no object for this preposition. You could say "They come into the house through the window." Then, I think that the Norwegian would have been: "De kamer inn i huset gjennom vinduene!" http://www.norskengelskordbok.com/en/dictionary-norwegian-english/inn
i is a preposition "in" Scroll all the way down to see a lot of examples of "i" used in many expressions: http://www.norskengelskordbok.com/en/dictionary-norwegian-english/i
"inn" is an adverb for "in" which describes how or where for a verb. "inn i" is a preposition for "into" http://www.norskengelskordbok.com/en/dictionary-norwegian-english/inn
"inne" is also an adverb used with a verb for in, indoors, or inside. "inne i" is a preposition for "within"
"innenfor" covers "into", "within" and "in" as a preposition.