You've really got some good points there, but in my opinion Oda didn't really show them deep enough. He barely scratched on the surface, like he did it with many other serious topics in OP too.
true. but i think it's mostly for 2 reasons:
1) hancock is a minor character and as such, cannot have a lot of panel time. other than the amazon lily arc, she has had very brief appearances. but these appearances are sprinkled
throughout a handful of arcs, which is why i feel she is a better-developed character than, say, viola or pudding - even though viola and pudding aren't bad characters per se, they just appeared for one arc only and have, thus far, had their characters revolve around that 'one central issue' closely related to their respective arc.
2) like with viola, hancock's sad past encompasses a lot more adult/inappropriate themes than pudding's (or katakuri's, or chopper's etc) and because one piece is intended for children, oda doesn't go deep into the details. he did with law, whose main theme was an incurable disease, and with nami, whose main theme was money - but that's because you can, sort of, explain these things to children without crossing the line. oda himself has said he can't write any details about viola and doflamingo's relationship in the manga - except the subtle violet/doffy nickname hint - because it wouldn't be appropriate for kids. i think hancock's backstory is the same.
But let's not fight about that, let's have fun with the game
i'm not fighting, i just explained why i don't believe in dismissing hancock as a one-trick-pony kinda character. btw, there IS a character in one piece who has NOTHING but the 'OMG LuFfY iS sO aMaZiNg!!!1111' gag to him:
if you compare him to hancock you will see she has a lot more going on. i highly recommend visiting the fanverse forum for soca's posts analyzing hancock, because his knowledge of the manga is superb and because he knows how to assess the context in which a character appears.
and Pudding is such a great foil to Sanji, she is what Sanji could've become if he stayed with Germa.
this is actually a good point. her family is in many ways similar to sanji's... and i think she made a fairly convincing villain. i hate her, though, as it's rare for me to like villains.
i didn't find the way oda wrote her heel-face-turn all that great, though (but i think all oda's heel-face-turns suck - even robin's). there was not enough regret there, oda chose to focus on sanji's kindness having an impact on her, but didn't focus on her realization that she has been an ugly and horrible person for much of her life (on the inside).