The analyses of a text #2

The narrator recalls his childhood, now of his early time in Salem House, the boarding school near London.
Mr. Creakle, who is the owner of the school, is extremely harsh toward his students.
Today Mr. Creakle keeps the house owing to indisposition.
The narrator's senior and friend, Steerforth urges the other juniors to make fun of one of the school's poor master Mr. Mell, who is one of the narrator's favorite.
They argues with each other when Mr. Creakle appears and Mr. Creakle now knows Steerforth called Mr. Mell a beggar and Steerforth purport that his mother lives on charity in an alms-house.

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"He is right, sir, without correction," returned Mr. Mell, in the midst of a dead silence, "what he has said is true."
"Be so good then as declare publicly, will you," said Mr. Creakle, putting his head on one side, and rolling his eyes round the school, "whether it ever came to my knowledge until this moment?"
"I believe not directly," he returned.
"Why, you know not," said Mr. Creakle. "Don't you, man?"
[David Copperfield by Charles Dickens]
I'd like to know the difference between "it ever came" and "it has ever come."
And I'd like to know "why, you know not" means "you don't know why."
Thank you in advance for your help.