Is "Meanwhile" a preposition? If not, what kind of sentence opener is it?
Meanwhile is not a preposition.
A preposition, as defined by Dictionary.com , is "a part of speech that indicates the relationship, often spatial, of one word to another".
Meanwhile is sort of like a transitional term that helps you state two actions which are or were going on at the same time.
In other words, it connects two actions together (unlike while, it can never be used in one sentence!).
Example: Molly washed the dishes in the kitchen. Meanwhile, Kathy fed her cat.
I'm not entirely sure if there's a word that describes the kind of sentence opener meanwhile is, but I am positive that it is not a preposition.
I hope this helps a lot! :)