Obviously there's a lot of confusion around the perfect tense, and we could fill volumes with descriptions and explanations of how it's used. However the thing to bear in mind is that it is basically a present tense, where present refers to whatever point in time the speaker is in when he uses it. With that as a reference, the perfect covers a timespan from a point before that "present" and up to it.
I have been writing this letter for an hour.
Here the "present" is now, and the timespan began an hour ago.
I had been writing the letter when Tony arrived.
Here the "present" is the point in the past when Tony arrived and the timespan is an unspecified period before that point. It simply describes what was going on before and up until the point that Tony arrived.
With the present perfect, you can mention when an action began, but you can't use any temporal that would suggest that you're referring back to the event as a finished, completed action at any point in the past, because, remember, it is for all intents and purposes a "present" tense - so it always refers to now. So *I've seen him yesterday - doesn't work because yesterday is over. However you can say: I've seen him today - if today is still today when you say it. At the end of today, you would have to say, looking back, I saw him today. (This may not be so in British English - I couldn't tell you. They often use the present perfect where we use the simple past).
The past perfect is different in this respect; You can refer to an action that continued or was valid up until the referenced point in the past as a completed action: He had written a novel in 2013. It simply cites it as an accomplishment of sorts - an action that was completed by that point in time.
Using the progressive simply implies that it was a repeated or ongoing action:
He had been writing a letter
= this is what he was engaged in up until the point referenced in the account, but not a completed action.
Also note, that the perfect is open ended; the action is understood to have started at a point before, continued or been valid through to the point reference (now or then) and may or may not continue. That is why: I haven't seen him today = not yet, not so far... but I may still see him at some point before the day is over.
I realize this is probably a vague answer to your question - more of a general overview. If you have any specific points you'd like me to clarify, that I missed, please ask. I've been teaching the perfect to Russians for a year and have gotten pretty good at it :)
Were I king = if I were king.
It is a present hypothetical. If I were king, I would make my birthday a national holiday.
Had I been king = if I had been king.
It is a past hypothetical. If I had been king, I would have given America its independence without a fight.
P.S. In response to the edited question which includes this:
How can I narrate a sentence which asks the listener to imagine that
... I was the King in some past time.
In other words, the truth is that I was never a King in the past but I
am asking the listener to imagine a situation in which I became King
in past.
Suppose I had been king.
That is, past perfect.
P.P.S. If the question is specifically about a hypotethical scenario in the past where you became king:
Suppose I had become king.
If I had become king, it would not have been by primogeniture, because I had a living older brother.
Best Answer
Out of your 4 sentences, three are correct. The only one that is not correct is:
Until is used for to specify a duration going on up to the point specified by what follows until.
In the following sentence, the action of studying for about 3 hours had been going on up to the point where the speakers was called.
The same sentence, but with before, is also correct, but there is a difference in meaning. The studying here also took about 3 hours, but it had already stopped at the moment that the speaker was called. So, the caller did not interrupt or stop the person studying:
The third correct sentence is:
The action was completed before the speaker was called. This is the reason that until cannot be used here, because that would mean the action had been going on up to the point where the person was called. The past perfect had visited here implies that the speaker had already finished the action of going to the store to buy food.