آفرین

It's better not to. It might sound like you are grading them. Use 'Mamnoon, darsetoon āli bood' instead.

No, it is secular today, although it has ancient spiritual roots.

'Āfarin' is neutral and standard; 'Eyval' is very casual slang used among friends.

No, use 'Tavalodet Mobārak'. 'Āfarin' is for achievements, not for getting older.

Common spellings are Afarin, Afareen, or Āfarin.

Yes, it is widely used in Dari and Tajik as well, with the same meaning.

Yes! 'آفرین به خودم' (Well done to myself) is a common way to celebrate a small personal win.

No, it is an interjection and doesn't change for plural subjects, though you can say 'آفرین به شما'.

It's a traditional way to give a 'perfect score' or high praise in Iranian education.

Yes, 'Afarin' is a beautiful and somewhat common female first name in Iran.

There isn't a direct one-word opposite, but 'Nafarin' (curse) is the etymological opposite, though rarely used in daily speech.

Only if you are the manager praising an employee. Otherwise, use more formal language.

Yes, people will shout 'Āfarin!' or 'Bravo!' at the end of a performance.

Yes, 'Āfarin به این دستپخت' is a very common compliment for a cook.