کلاه سر کسی گذاشتن

No, it's not rude, but it is an accusation. Use it with friends or when complaining about a third party, but be careful using it directly to someone's face unless you want a confrontation.

Yes! If a friend tells you a small lie as a joke, you can say 'Saram kolah gozashti!' with a smile.

'Gool zadan' is general (to fool). 'Kolah gozashtan' usually implies a more elaborate scam or a financial loss.

You would say: 'مورد کلاهبرداری قرار گرفتم' (mored-e kolah-bardari gharar gereftam).

90% of the time, yes. But it can also be used for emotional or social deception.

The 'hat' (kolah) stays singular even if many people are tricked: 'Sar-e hameh kolah gozashtan'.

Only if you are talking about a past experience where you learned to be careful. Don't use it to describe your own tactics!

There isn't a direct opposite idiom, but 'Sadegh boodan' (being honest) is the conceptual opposite.

Yes, it is common in Afghanistan (Dari) and Tajikistan as well, though with slight accent variations.

Yes, kids use it when playing games if someone cheats.