"Who was with you?" and "Who were you with" are equally grammatical and natural, and carry only a very slight difference in meaning.
"Who was you with" is ungrammatical for most English speakers (since it corresponds to "You was with X").
The claim "prepositions in questions usually come at the end" is wrong without some further specification. If the preposition logically precedes the question word (who, which, what etc) then in colloquial English it usually goes to the end: that is, most people would say "Who did you go with?" rather than "With whom did you go". But this does not apply when the preposition goes with a noun phrase that is not the question term ("with you").
Of what audience?
Scholastic English traditionally regards starting a sentence with a coordinating conjunction to be an error. For that audience, a compound sentence is more acceptable:
Maybe we can do something about it, or is that impossible?
You might know this general pattern by the name tag question. If not, you may still recognize it as a compound sentence. Despite the fact that the question no longer forms a sentence on its own, it remains an independent clause and the sentiment remains much the same.
Or, does it?
In conversational registers, it's common. For example, I see such questions standing not merely as separate sentences but as independent paragraphs in my professional e-mail correspondence. I've used it myself, just as I've used it above, to mark a sharp transition in the body of the text. As a tag, that question cannot serve that purpose quite so clearly.
Conversational registers are far more lenient regarding sentence fragments than formal or scholastic writing. For example, the first line of this answer is a prepositional phrase. It's a grammatically sound prepositional phrase, but it certainly does not constitute a clause in formal writing. The fragment makes sense here only because you provided the context that makes the question sensible: "Of what audience [do you want to ask such a question]?"
It's not impossible, it's not even unlikely, but it's not appropriate under every circumstance.
Best Answer
Whether or not it's "grammatically correct", at least here in the US, people say this kind of thing all the time. Something like:
is definitely a question, one that is asking for permission or approval. Asking:
is the same as asking "Can I help you?" but with more uncertainty whether you want or need my help.
That being said, you shouldn't confuse this with the "rhetorical question" structure that is a statement:
This is not a question, but a statement saying "It would have been a good idea if you had ..." Similarly:
Prefacing a question with "maybe" is informal, and is more common in certain cultures and dialects than others, so I would be careful using it -- especially since the regular question structure is always acceptable.