Swedish has gendered nouns: common gender and neuter. Common uses "en," neuter uses "ett." If you look at the browser version, it has a very nice grammar section under the Basics lessons that discusses gender in Swedish.

No. It is not an O-sound, but a long A-sound a little different to the one many are familiar with. It's the same sound you'd find in English words like "bar" and "far".

Yes

Swedish doesn't use the indefinite article for people being things like professions. It's required in almost all other contexts where English has it.