Is through an interjection

Answer 1

No, the word 'through' is not an interjection.
The word 'through' is a preposition, an adverb, and an adjective.

Examples:

Let's take the shortcut through the park. (preposition, 'the park' is the object of the preposition)
We came through without a scratch. (adverb, modifies the verb 'came')
Highway twenty is the through route. (adjective, describes the noun 'route')

When I finish this sentence, I'm through. (predicate adjective, restates the subject 'I')