if "minus" is conceivably an operator, as in "the band reunited, minus one member", you would use the singular.
Which is preposition? the 'as' or 'in' or both as a whole is preposition? In English, can two preposition be used one after another?
It often helps to complete such phrases, because filling in the omitted (or imagined to have been omitted) words can make the construction clearer.
In this case, as in stands for as [it is] in:
Then it is easy to see that in is a simple preposition, and as a conjunctive adverb. As Reg said, you could functionally consider the whole an adverb. Just as you could consider black coffee a noun. (I don't quite see the need for this invention, as it can be cut off by Occam's Razor.) Note that this is also the only satisfying way of analysing "as if": he fled the city in a great hurry, as [he would do] if he were chased by bears; no wonder, after such a scandal. Note that it is completely normal in countless cases to leave out words in comparisons with as and than.
In is a preposition, but as in as a whole acts as a conjunctive adverb, according to Wiktionary.
As to your second question — yes, in English, two (and more) prepositions can be used one after another. For example, "Barack Obama stepped out from behind his teleprompter". Or, borrowing a leaf from ShreevatsaR, "Daddy, what did you bring that book that I don't want to be read to out of about Down Under up for?"