DC is listed on IMDb as creating the characters.
I suspect that will come as a surprise to the estate of Art Wallace, who created every character up to Barnabas (and there's even a court ruling to back that up). But then, who knows who even added that credit to the site...
As for fitting in a sequel (or sequels) to a successful film, projects with an already established audience are hotly desired commodities in Hollywood, so stars, directors and studios manage to find/work out ways to do them. For example, considering his schedule is always so full, I suspect Depp already had several films in his pipeline before it was decided to do the first two Pirates sequels, yet they were quite successfully shoehorned in.
With Burton, though, it isn't always the case that the same director helms the sequels...