Dear Pructus, I'll take Section A only. 1. You need "he will leave" because you are referring to the future.
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pructusHow about if A-6 is changed to: I know if he leaves or not., from I know if he leaves.Then, the sentence becomes good and grammaticalActually it doesn't become grammatical because it was already grammatical. You've just changed from one grammatical sentence to another grammatical sentence.
pructusUnderstanding "I know if he leaves." to mean "I am always aware of his absence whenever he leaves." seems to take the structure as “If he leaves, I know"Correct.
pructusIf this is so, then is your explanation like below?Even though "I know if he leaves." can mean “ I know if it is a decided fact that he is leaving in the future”