So in Jn 19:14, the "Day of Preparation of Passover Week" is the Friday of Passover Week, Nisan 15, which is the first day of Unleavened Bread,
and the same date of his death in Mark 14:12.
That doesn't make sense. You don't prepare for Passover week on the day of Passover. Why? Because Passover is part of Passover week. You can't exclude Passover from Passover week. Thus, John is talking about something entirely different.
More so, we know from John that Passover had not yet come. Why? John 18:28 states specifically that Passover had not yet occurred.
Family = relatives.
Those with him were not his immediate family, but were cousins, etc.
That's not what scripture states. Anyway, we are told that his aunt, and cousin, know who Jesus really is. Luke states this. More so, why would his family not know who he really is? If he was born of a virgin, from God, his family, meaning cousins and all, would have heard about it.
Yes, Jesus is the Son of God, but he did not make that known to his relatives, for the same reason he hid it from the masses.
How? How did he hide the fact that his father was God, and not Joseph? I mean, we know from Luke that Elizabeth knew. John obviously knew. Do you think that Mary would have lied to all of her family? She wasn't married, and all of a sudden, she's pregnant. Obviously, she is going to say something.
So that such a scenario would not occur, Jesus promised them empowerment to recall and understand all things correctly (Jn 14:26, 16:13-15, Lk 24:48-49).
Your latter day novel speculation is overturned by the NT testimony.
Actually, NT testimony doesn't agree with itself all of the time. As in, it contradicts itself.
Second, it doesn't matter if Jesus made such a promise, as the people writing down the accounts weren't there in the first place. They were getting the accounts from second, third, or even more hand accounts. His disciples did not write a single word about Jesus, as far as we know. More so, we are told that his followers, or at least some of them, were uneducated, and thus would not be able to write.
Finally, if we stick to the tradition, Mark was based on oral tradition. It wasn't an eyewitnesses, it was someone who was copying what Peter said, long after the fact. Luke, by tradition, was a traveling partner of Paul, who was not even a disciple, and thus, was reliant on oral tradition.