"Help yourself" is used when we invite someone to take something. It means "Don't wait for me to serve you" or "Don't wait for me to give [something] to you". Those cakes look nice. Help yourself! We've got more than we need. Can I borrow a pencil? There's a pot full of them on my desk. Help yourself. "Be my guest" is used when someone asks if you mind if they do something . We often say this in reply when you are more than happy for them to do it. And yes, it's sometimes slightly sarcastic, because it may be very obvious that you want the person to do this. I just came across an example today of a situation where I used 'Be my guest' here on italki. I had highlighted a problem there was with a bogus native English teacher, and another member asked me if it was OK with me if he reported this person to italki admin. My response was "Be my guest", meaning 'Of course! By all means!'.

They can be interchangeable but it depends on context. Be my guest is the right term when showing someone into your home. Help yourself when offering an open bottle of wine. The other way around would be a little odd, but would be understood if spoken by a non-native. Both phrases can be used sarcastically

If you want a glass of wine from a bottle that someone has just opened, you could ask: "is it Ok if I have a glass of wine?". That person could then say "certainly, be my guest", meaning, "sure, go ahead..I am happy to share it".. They could also say: "that's fine, help yourself".. Or, you could say "may I sit down?", or "may I join you?" the answer would be.."of course, be my guest".. You would not really say "help oneself" in everyday speech. It would be best used to describe a situation such as: .. .."there was a large buffet table in the restaurant, it was possible to help oneself to some tasty dishes".. Hope that's useful!