It is difficult to see from the planning drawings (as there are so many and made to confuse IMO) how the existing 'tip' access road is going to change to affect the main road. But surely, for a 'public opening' event for the Mill, some volunteer road marshalls and notifying the local Police would suffice?
The plans for the access road look like they are going to go ahead and apart from 'red tape' and 'official bull$hit' I can't see any material reason the Mil shouldn't operate as normal. It's not like they are planning a new junction to the M1!
If it were me, I would say 'bollox' to them and just carry on and deal with the Council or Highways if they have a problem.
Persimmon and the Council want the housing estate, Persimmon want the access road (presumably as it is the cheapest place to put it). Doesn't mean the Mill owners' lifestyle should be turned on it's head because of that.