1) Oda explained One Piece world's flintlock pistols are a variation to the single bullet pistols in RL, holding up to 6. We've never seen a character actually rely too much on the gun, though, never shooting more than one or two rounds before being knocked off. Usually it's a much different gun variation, normally custom made, that is used when a character relies on guns for their attack, like trick bosses, with bodies made of guns, for example.
2) This is simply untrue. Her new Feet stomp as been used countless times to short out a great number of enemies of the crew, not just herself. In Punk Hazard, she uses giant hands to hold back the children from eating the candy, while she stood at a different spot. She created a clone of herself, in Fishman Island, to stop the fight between Luffy and Jinbei. And many other instances.
In your example, we can hardly fault a character just because they didn't do what you wanted them to do. Who's to say she could actually get enough concentration to pop hands at such a long distance, hardly seeing them, and while being shot at?
3) It's a recurrent gag in the show that, while the extraordinary body of Franky, supposedly the dream toy of every male, is looked at as childish by the women of the crew, going as far back as Franky's megazord idea back at Triller Bark, and Robin's reaction. They acknowledge it's strength, but frown upon it's maturity. It is, after all, a comedic series.
4) Agree, half and half. Though I do consider him dead. We can not forget it is a child's show, especially so early on the series. And sometimes, these happy endings do exist. They don't bother me as much, as I consider them as dead, as I said. It's all about reading between the lines.
5) Case you're referring to the "endless" cyclone of his, he claims it will follow them to the afterlife, chasing all the way to hell. It doesn't really state it kills them. In fact, Zoro's never been to kin on the idea of killing people, same as the other crew members. Oda subscribes this as having the character's defeated mentally and physical, without the need to go the extra step of actually killing them, which is a good concept for any action movie who focuses on storytelling rather than gratuitous violence, and is an even better concept for a children's series.
6) Meh, you're just knit picking. Usopp's character was estabilished as "weakest" and "cowardly". He's still a Straw hat, and has the inherited courage to surpass his fears when his friends need him the most, as well as the resourcefulness of a very smart person. But he is the closest thing to a normal human, to us, there is. It's almost as if we could live in their world, and we'd be very similar to Usopp at that.
And who is there to say he didn't get scared all the time in the island? He learned to harness it's power, and he managed to escape much stronger, but it doesn't mean he didn't fear it all the same.