The reason we're all here is to help each other, so ask away.
You got the meaning right, so I'll just help with the analysis. πεπεμμένοι here is not the subject but is the complement of the implied subject that goes to back to ξένους. The reason for εἰσίν is that in the passive of the perfect tense, instead of there being a single verb, you use "to be" plus a participle. For example, if the question had been "where they are going", it could be something like
... ὅποι πορεύονται
so πεπεμμένοι εἰσίν as a group functions like πορεύονται.
About ξένους being accusative, the key thing is that for the most part, the case of a noun is determined by its role in the clause it's in. So ξένους is in the main clause and is the direct object of ἐρώτησον so it's accusative, but πεπεμμένοι is in the ὁπόθεν-clause, and there is modified the (implied) subject, so it has to be nominative.
And about τοὺς οὐκ ὄντας Ἕλληνας, here because of the article, it should be "the strangers who are not Greeks". It would be"not being Greeks" if there were no article.