Abandoning wire in attic space
As far as I know there is no code saying you cannot leave de-energized wires anywhere you want to. When the inspector said that "capping off the wires in this new void space is not allowable" he probably said it with the assumption that they would still be energized. I think it would still be responsible to label them with a tag saying something like "dead circuit, do not energize".

If I'm wrong then the Sparkys on this forum will surely correct me.