"ar feadh" is an expression approx. "for duration / length of" and is the one making genitive (nóméid, rather than nóiméad) necessary. In the translation the expression is shortened to "for". "ar" is actually "on" in English, and not for.
Thank you for pointing out the approximation of the translation. Ar feadh, would, be a phrase which shifts its Béarla meaning dependent on context? For duration it would appear to mean a while, the fullness of the moment or a until the end of the time?
She sleeps until she wakes.
Yes. This is common with compound prepositions - see here http://nualeargais.ie/gnag/genpraep.htm#abgeleitet
"They" don't change this because Duolingo was designed to use Text to Speech engines to read out the exercises - it is impossible for the audio to not match the text. Irish doesn't have a text to speech engine, so there are mismatches between the text and the audio, but there isn't any procedure for fixing those mismatches, because those mismatches aren't supposed to exist. The people who do care about the mismatches are the people who developed the course, but they don't have the necessary access to the system to change it, and the people who do have that access don't want to make those kinds of minor changes.
I definitely wouldn't describe it as "ds", but slender consonants (consonants next to the slender vowels e and i) have a different sound compared to their broad counterparts (the same consonant next to a broad vowel, a, o, u). This difference is most obvious with s, and is often more obvious in Ulster Irish than in Munster Irish.
The slender d in nóiméid doesn't sound the same as the broad d in nóiméad, and the sound that you describe as "ds" is the slender d.