It doesn’t matter about the orders, Luffy would literally be letting the only person to kill a nakama/family member walk away free.
Luffy has to fight Blackbeard, and he has to fight Akainu. Just storywise, ignoring feats, if either of those battle doesn’t happen it’s drop worthy.
Do you remember how badly Ace’s death fucked up Luffy?
He literally almost gave up on his dream and wanted to kill himself. It’s the the only time in the entire series that someone killed one of his nakama/family members. Guess what the main theme of the seiries is? NAKAMA
If Akainu got...