My big question is whether Red Hawk is Luffy manifesting Ace's powers somehow, like the scene seems to imply, or if it's just some mundane method like Sanji uses.
Or if it's Luffy using Haki in another way that doesn't make sense with how it was originally described.
I don't believe that's the case at all.
After all, Pacifistas, which devastated the group pre-timeskip, were being one-shotted afterwards.
The New Fishman Pirates, while not a truly credible threat, are still doing better than that. I'd say these guys are all tougher than CP9 or the Thriller Bark pirates, it's just the Strawhats have gotten so much stronger in two years that they're making it look easy.
I understood it as it's the same sword he had before, the Longarm Tribe just sharpened it with some high level of skill.