Translation:The women eat a sandwich and they drink milk.
Use of singular implies one sandwich per person. In a more general context if each person has or can have only one of an item the singular is always used. For example "their heads" in English goes into Irish as "a gceann" (their head) - "a gcinn" (their heads) would sound most strange as it would that they had more than one each. The sentence as it stands is correct.
In Philadelphia, it's common to have a hoagie the length of the table sliced into small segments at catered luncheons. https://qph.fs.quoracdn.net/main-qimg-50af4cde5151b5f2674aaa9f67cd5254
In both Ulster Irish and Connacht Irish, certain worlds that are spelled with an "n" have an "r" pronunciation. (cnoc is another example that crops up on Duolingo). In Munster Irish, and for most people who aren't native speakers from Connacht or Ulster, the "n" is pronounced as an "n".
Almost right - it's ag ithe. And there is no eclipsis, so you were right the first time with ag ól bainne. (After ag+verbal-noun you use the genitive, but in this case, the genitive of ceapaire is ceapaire and the genitive of bainne is bainne).
The important point is that Irish and English both differentiate between the simple present (itheann na mná - "the women eat") and the present progressive (tá na mná ag ithe - "the women are eating"). Some other languages don't make that distinction, but you can't translate the simple present in Irish or English into the present progressive in English or Irish.
Because the Subject comes before the verb in English, it is technically possible to skip a repeated subject (the first subject is still before the second verb). But in Irish the subject comes after the verb, and you can't have one subject apply to both verbs. Siad refers to na mná.
The speaker is clearly saying siad, not sibh. If you think that you hear sibh, that's a reflection on your abilities, not on the speakers pronunciation.
You are also completely wrong about the pronunciation of mná. In both Connacht Irish and Ulster Irish words like mná, cnoc, gnó and de ghnáth are pronounced with an r sound. Irish speakers from Munster and most non-native speakers pronounce these words with an n round, but this speaker is from Connacht, and her pronunciation of mná is absolutely correct.
Perhaps if the speaker didn't drop off at the end of the word is would be more clear. I listened to it several times after I got it wrong and still couldn't hear it clearly so when it cam up again at the end I just typed what it said the correct word was before. There was no need for you to belittle my abilities.
I have not seen anything in Duolingo that tells me what province the speaker is from and it wouldn't help to know that anyway since I am not familiar with the different dialects. You seem to like trying to make non-speakers of the Irish language feel inferior instead of trying to help. Why bother replying if you are only going to cut people down? The person I know who teaches Irish is a native of Ireland, from Dublin, and he is a teacher. Dublin is in the province of Leinster and you did not mention anything about those from Leinster and how they would pronounce mná. So don't emphatically state that I am completely wrong if you don't know the facts.