It is neither. Alas is an interjection (utterance). It can also be considered a sentence connector, e.g. There were, alas, no survivors.
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It is an adverb. It cannot be a preposition. It may be considered a noun in some uses.
On can be an adverb a preposition or an adjective. adverb - Turn the lights on. adjective - The switch is in the on position. preposition - I'll see you on Sunday.
No. Almost is an adverb. It is not used as a preposition.
It can be either. If it takes a noun as an object, it is a preposition. It if stands alone or with other adverbs, it is an adverb. It can, rarely, be an adjective.
It can be either. If it has a noun as its object, it is a preposition. Standing alone, it is an adverb (and possibly an adjective). It can also be a conjunction.