No idea on the level six and upping everything a HD idea, sorry.

Having the Witchwood all figured out initially is the minimum I'd recommend, since it's unlikely but possible they'd blow past Koth and make it to the bridge in a session. The most important things are the primary antagonists in the form of the dragons and the wyrmlords.

A generic world would require less work to set up because you can just go "welp, here's the vale" whereas with a more rigid (at least in regards to hordes and armies prowling about), established setting like Eberron you've got to figure out where to put it and have some level of pressure to take into account setting fluff/mechanics. So, it depends on whether having to do less work in making the adventure fit with the world or having some established canon/fluff to flesh out the world is preferable for you.