Welcome to the forum, Joe.
No it can't. Where did you get that idea?But usually the context will make things clear, and very often the apodosis is the result of, or the thing that happens/ed after the event described in, the protasis, so the order of events is known, just as in the first two example sentences we use the apodosis to determine the time of being you in the protasis; for the above sentence, had asked can be reduced to asked, without ambiguity.Correct. Only 'had known' is possible in a counterfactual conditional.
Here comes the question:
If he had known/knew this, he would have been very sad.
My opinion is, one option must not be replaced by the other,