It's unclear what the developer wants to do with the house, but my first step as an art/arch historian would be to collect as many black-and-white photos of the house interior as they can find, then identify any furnishings that are original to the house, and set them aside for preservation/reinstallation. ;-)
An historical Plas destroyed by cowboy developers, the council refused to sell it to a local couple, hence the state its in today. Family origins from 843ad,. It was sold by the Newborough family in 1951, and became an Agricultural college. Family today runs the Rhug Farm near Corwen.
Gosh, all he could find in that large house was an occasional Octagonal table worth about £100! Somethings wrong. Certainly not going to make one iota of help to refurbishment costs