This is specifically meant for TheHappyCaptain. Normally I would PM but your review seemed really detailed so I thought that I should at least return the favor. You don't have to read if you don't want to as it is really meant for him but it would help if you had the same views as him.
I'll definitely agree that the reason was weak. However it's tough to write up any sort of argument which would make Luffy leave the crew of his own free will. I mean if its blackmail, then yeah that could work. But to make Luffy really leave. That is hard. I once had a good reason for Luffy to defect from the crew but I already used it up in my other fic One Piece Shippuden. I just cannot think of another. You could check it out if you want. I think you'll agree its definitely better than this one.
The argument is not the basis of the story. The basis is the war. The argument was just needed to get the plot forward. Like a catalyst. The story does not revolve around the argument. If it was then the story would be a Hurt/Comfort one, not an adventure one.
I know Luffy is someone who get's yelled at a lot and should take it personally but it is hard to write about any real confict between the Straw Hats.
Zoro respects Luffy that's true. But during the discussion of Ussop's return Zoro did say that he would leave the crew if Luffy let's other step over him. And that is what Luffy did. He let Nami and Sanji step over him and left. The guy Zoro respects even nearly died for at the hands of Kuma just left over such a petty reason. Why should he defend such an individual? Nami and Sanji not apologizing. They do feel regretful and they might have apologized had he returned to the crew. He did not so they obviously cannot apologize.
I know a lot of people joined the crew specifically because of Luffy but I do not see them as the type of people to just give up if one member leaves even if it's their savior or whatever. If one does leave, they would try to get that person back. If the person doesn't want to come back, they'll have to move on. When Ussop left the crew, they were able to still function though depressed and all.
Luffy joining the army. Well a spoiler here. Luffy is able to get people to follow him despite their extreme situations. Likewise his father has a similar ability to get people even someone like Luffy to follow him. Also without his crew giving him strength, he is more prone to Dragon's brainwashing. That among other reasons to be revealed later in the story.
Luffy is probably the only Straw Hat who would act OC. I definitely plan to have everyone act themselves.
Normally I would just PM certain people but your review seemed very informative so I figured I had to return the favor.