Luffy ALREADY destroyed half of marineford and punched Garp out of his way
And the funniest part, forget about Garp letting Luffy go... I'll be logical and not ignore the fact he let him go...
The funny part is... a VERY TIRED exhausted Luffy with TONS of wounds, who has been fighting SINCE Impel down with NO REST.... And a very serious FRESH Sengoku who used his DF + his attack was AIMED to execute BOTH Ace and Luffy according to Sengoku's dialog... and Luffy took the attack HEAD ON... and was FIIIIINNNNNE
I don't need to tell you if Luffy CAN DO what Shiki did in Marineford... becuase Luffy was actually SIMILAR in a lot of ways to begin with in Marineford hahaha
This is so funny... you asked a question that was ALREADY answered in Marineford arc
For your second question, I honestly read it THREE TIMES and still don't understand it... rephrase it again or explain what you mean because I didn't get it