Well it seems you have all the correct responses...so...nothing more to add ...except...people that use that expression are usually using it in a joke, figuratively,...or showing lack of education.

"I ain't" is used mostly in the US (and in 'cockney' (London) slang) in a colloquial context to replace the words " I am not..." " You are not..." etc (all forms of 'to be' in present tense) but is not really correct.

Forget "ain't" and (almost) all other slang. (Also don't learn obscenities.) They're just some of the things that non-native speakers don't need.

Yes it is used sometimes in speech. But you are better off using the phrase " I am not" instead. And like Averill stated, it is never used when you write.

Of course it is. But you shouldn't practice saying it, because it isn't proper. You should always practice "proper" speaking methods. Oh! And don't ever use "ain't" in writing. Not good. Well, unless you're asking a question like this one.