I agree with most of your post, and I actually did enjoy this arc as well, though I am currently suffering from Arc fatigue. I think it'll be better in chapter form. (There was a lot of potential BS in this arc, but I am pretty open about what does and doesn't make sense in One Piece, and about why people do what they do, so a lot of it I don't mind as much as others.)
I do want to disagree with the two parts of your post I quoted above, though.
(1) While I agree that Luffy has more tricks up his sleeve, I'm not sure a gear 4th gatling is one of them. As far as I can tell, gatling works by throwing his arms behind him slightly and then launching them forward rapidly, taking advantage of his rubber body to "snap" his arms out after stretching them back. In gear 4th, however, he doesn't seem to use his body in the same way. His punches are launched by pushing his fists into his body, and then releasing them so that they fly out of him. So far, with both kong gun and king kong gun it looked like he had to push his fist into his arm and that it took a little bit of time to do that. Not much time, but more than between gatling punches. So, a gear 4th gatling MIGHT be possible, but if I understand gear 4 and gatling correctly, I don't know that it would actually work.
(2) I posted this a while back, but basically: Zoro bashed his katana against birdcage. He slammed his katana against birdcage. If he used axes, this would be enough to show he can't cut birdcage. However, katana are not bashing/slamming/chopping weapons. They are slicing weapons. Their ability to cut comes from a certain motion in which the blade is dragged against the target. He did not do this, and so his bashing tells us nothing about whether or not he could cut the cage. It would be like saying my gun can't pierce this window after bashing my gun into the window. Well… that isn't how guns work... try shooting.
Though I like this arc, this is one of my biggest problems with it. Zoro didn't even try to cut the cage, which I could forgive if not for: (A) "There is nothing I can't cut!" and (B) "How do you know it can't be pushed if you didn't even try!"
(I see a lot of people respond to (A) with: "But he told the giant the cage would be gone by the time he was awake, and he was WROOOOOOONG." Okay cool, but that doesn't mean he didn't honestly think it would be gone. And even if he CAN'T cut the cage, that doesn't mean he doesn't think that he can. Then he nearly immediately made the (here germane) comment: You don't know you can't unless you try. So, here's some facts: He thinks he can cut anything, and he didn't try to cut the cage (not even by bashing his katana at it). Why?
Personally, I think because the arc is too long as is and it would take too much time to show him try, and then have it just regenerate or something like that. Depending what Oda could think up about how the cage works, it would take at LEAST 3-5 panels, and probably more like half a chapter (or longer!). I think that's why we didn't see Zoro even try to cut the cage, despite thinking he can. It wasn't plot-convenience, per se, but rather a king of story-telling format convenience.
Interesting with your thoughts on Gatling. I agree that does seem to make sense that a certain level of retraction is necessary and a little bit of time. But I think the gatling is still possible for the following reason. When Luffy launches the punch and stretches his arm, on the recoil the arm will fly back to his body at high speed, at this point he can easily retract it into the compressed firing position and shoot it again, like a rubber band or a rebound and I think he will do this. It's like starting the punch he must contract his arm since he has no speed build up, but after he has launched a punch and his arm is retracting towards his body he can use that speed to cause the compression and retraction and fire another punch very quickly. I think his Gatling in this version is island level and might be what destroys fishman island.
2. Regarding Zoro I just personally believe from everything Zoro has shown that it's either one of two reasons. One was that he didn't have the power to cut the cage and he knew this instinctively and chose to slow it down or two, was that due to his pride and expectations of his captain in the new world is that he decided to NOT cut the cage since it was Luffy's battle that was underway with Flamingo and this would be interfering with your Captain's battle, which NO Vice Captain would do. The second explanation is fine to me and I don't see a problem with that. I do believe if Luffy failed Zoro would actually attempt something like Asura.
In relation to the first reason it could be that bird cage is that powerful, being a special preparation technique of Flamingo, a pirate who has been on the seas and in the new world perfecting his powers for many years. I do think the second explanation is more likely and Zoro was holding back. I don't have issue with this because Zoro has observation haki and his Captain said he would deal with it. That's kind of a sticky situation since a few of the citizens were close to dying to the birdcage save for Mansherry's healing but overall I'm fine with it.