Lots of good ideas here!
As I see it, the more adventure sites that is kept unmapped beforehand the better. It is good to have something to drop in the path wherever the PC:s go.I will however map the big powercenters that will affect their surroundings before the campaign starts.
The Hollows, Vale of the Dead and the Spire of Quetzel-locations could be almost anywhere. Kept unmapped. (Spire of Quetxzel itself would do good as the castle in lake Varda yes).
Grindbone suits best roughly in the middle of the map I think, so that everyone easily can come here to trade with slaves. Kept unmapped.
Ravenhole suits best in Feulamark, but could just as well be anywhere. Good location to just throw in when adventure is needed. Kept unmapped.
Weatherstone could also be almost anywhere (Zygofer could have felt the presence of Merigall better here by som magical circumstance rather than geographical distance). Perhaps not to far to the north or east. Kept unmapped.
Amber’s peak - either in the middle of Margelda plain (Ah20), or in a custom location closer to the alslenes, northeast of The Shroud.
Eye of the Rose - Varassa
Stonegarden - either the fortress at E8, or at a custom location in the mountain range where the Canide dwarves are marked.
Stoneloom mines - in the middle of the Arina forest.
Haggler’s House - important location to map early, since the rust brothers most likely will hold the surrounding countryside in a tight grip. I think the fortress at P47, or close by, so that the whole southwest corner of the map becomes dangerous.
Farhaven will need to be mapped. Maybe an enclave along the ailanders at the southeast coast of Margelda? Or at the south coast of lake Varda?
Scarnehall with its big statue in the mountainside, which would be seen by people passing the Crombe lands in northeast. One of the castles here.