Not interchangeable. “Are you still not ready?” suggests that you might have checked on the readiness previously. “Haven’t you got ready yet?” suggests an unmet expectation. So if someone is going to take a date out to dinner and shows up at her house and learns that she isn’t ready, he could say the first but not the second.

I agree, these can both be used interchangeably with a native speaker.

Neither sentence sounds natural. See comments from others who have responded for more natural sounding choices.

Both work ok. But both sentence are constructed formally. A native speaker is unlikey to be in a formal setting when they are asking someone if they are ready. It would be informal. An informal version of that sentence would be "Are you ready yet?" (polite request for an update on their 'readiness') or "You're still not ready?" (expression of frustration at their 'not readiness')

As Jake said in his comment, both will be understood by a native speaker but the phrases he suggested ("are you ready yet?" and "you're still not ready?", or, in my experience, "have you finished getting ready?") do sound more natural and are more commonly used by native speakers (at least speakers in my area, the Northeast of America). Both of the phrases you mentioned, however, are correct, understandable, and interchangeable with one another. I hope this helped, good luck with your studies!