Αμφίψωμο is the Greek word for σάντουιτς. Σάντουιτς is the borrowed word. Αμφίψωμο is not really used, but we added it for completnesse's sake.

You use μια for nouns with feminine gender and ένα for nouns of neuter gender (as well as, in the accusative, masculine-gender nouns that begin with certain letters).

It's like the difference between German ein, eine or Swedish en, ett.

It is not said in Greece - at all. According to the school grammar book taught all over the country, the vowel contractions in such verbs are compulsory.

You will hear it in Cyprus though. Very frequently. Standard Modern Greek is taught in Cyprus but there is great difference between the spoken dialect there and the 'official', taught Greek that is mostly found in written format, if I am not mistaken.

So it is part of the vernacular for a number of native speakers, but, if we go by the books, it is not really a proper term.
Edit: To clarify, you would be understood in Greece just fine, but it would sound like you have not mastered conjugations.

What was the entire sentence that you wrote?