"May I ask if..."

Such a degree of politeness isn't normally required in such a question. When it is, anyone who answers it with your second alternative is being intolerably perverse, and you should dissociate yourself from such a person at the earliest opportunity.

I'd suggest this:

If I may ask: have you seen it all?

Here the valid answers are:

1. No (I haven't seen it),
2. Yes (I have seen it),
3. No, you may not ask.

You have only one question, but retain the polite possibility that the question is out of bounds.

(Edited to correct error pointed out by @jwpat7)

Are you asking how to respond to a question like this or how to ask it?

What you have quoted is really asking two questions at the same time. A better form might be "If I may ask, have you seen it all?". That would likely avoid the flip response "Yes you may ask" and still allow the other person to say "I'm sorry, but I'd rather not say".