For the purpose of determining your nppr, ownership of the house in which you reside is irrelevant. The house is still owned by the estate. You are concerned about residence in your old house. You need to declare to Revenue when you moved out of your old house, where to is not relevant. It is unlikely that Revenue will ever ask for any proof of when that occurred but in the event that they do, it could be useful to hold onto the first sets of bills, bank statements, whatever could be admissable.
To my mind you moved into the house when it became your home. This could be the first night you slept there (even if occasionally sleeping in the old house) or when you moved in your furniture.
Sybil