It used to be centuries ago, when donna was a honorific: now it's only used for the Lady (i.e. Mary, mother of Christ). And the singer. The old usage survives in some dialects (etimo.it says for the wife to address the mother in law), and same for "donna", e.g. "donna Giovanna": the masculine "don" is internationally associated with the mafia, but in Italy it's mostly reserved for priests.
Yes. A masculine word starting with 'S + vowel' needs un. (It's called an indeterminate article.)
A masculine word starting with 'S + consonant', e.g. 'sp' or 'st' (think 'stivalo'), or 'z', 'gn', 'bd', 'pn', 'ps', 'x', 'ch' — needs 'uno'. In this case the determinate article ('the') is 'Lo'.