> Urdu script can distinguish them, by adding a diacritic to the ا, but it's usually omitted
Not in my experience and certainly not as a student. I'm not an expert but at my school اس for something that sounds exactly as /ɪs/ would be considered completely invalid. Only اِس would be considered correct. Actually, I think اِز (more like iz) might be more correct phonetically.
In school, perhaps so. But looking at something like https://ur.wikipedia.org/wiki/اردو, there's scarcely more than a handful of short vowel marks (zabar, zer, pesh, or fatha, kasra, damma as an Arabic speaker would call them) anywhere on the page; the vast majority are left unwritten.
While I'm not 100% sure, I think it _might_ be the case of writing Urdu online vs on paper. I'll try to grab an Urdu book or newspaper tomorrow and see how it is written. Thanks for the insight, this was very interesting.
Not in my experience and certainly not as a student. I'm not an expert but at my school اس for something that sounds exactly as /ɪs/ would be considered completely invalid. Only اِس would be considered correct. Actually, I think اِز (more like iz) might be more correct phonetically.