Not a teacher.
'Much more better' is wrong. It must be 'much better'.
This is true, however, when English is used colloquially, you'll often find examples like this. Consider it to be a dual modal, used to convey stronger meaning. On a scale it would look like this:
Much more better > much better > better
A prescriptivist would say that something can't be "more better" and that much better conveys the same meaning as the original. That's why it would be marked wrong.
As for the second question, after a door has been opened, it can't be in a state of "more open," so to speak. The door may certainly open further, but it is still open. It's comparable to the idea that a woman who is 9 months pregnant is not "more pregnant" than a woman 6 months in. She is just further along in her pregnancy.