The simple answer to both of your questions is 'no'. You can't generalize sentence X to always mean Y and always assume people mean the opposite of what they say. It really is impossible to say without context what the other person actually means. It all depends on how they say it: tone of voice, body language etc. In the end, it's up to you to decide whether they are being sincere or if they actually mean something else.

I've seen this chart going around... what the British say and what they really mean. :) Take it as a piece of humour. It could be false... it could be true. Don't take it at face value. Are you paying enough attention? Then you'll have no problem understanding.

When someone says, "That's interesting," after hearing you speak, it can mean different things. It can mean that they really do think what you're saying is interesting & they are intrigued & thinking about what you said. It can also mean they think your idea is uncommon & they do not agree with you or look down upon your thoughts. I think it depends on the tone of voice used & what is said after. I've usually experienced that response with consideration or dismissal of my ideas. "Good try" & "that's a brave attempt" are also phrases that depend on the tone of voice. People tell you this if they really do admire your effort or courage at accomplishing something or use sarcasm to mock your efforts.

very interesting:) it was a joke:) i hear first time but if this is true ,really interesting.