As stated above, the DfT have refused to allow ATOC to amend the wording. Of course they want to change it!
The way I see it, there is nothing for Derby to lose by trying to obtain a reservation from the ticket office. In the eventuality of a conflict, it may give credence to their argument if they can show that the combination was ratified by a member of staff in the ticket office.
Buying the tickets online is a good idea. The barriers at St Pancras High Level aren't Oyster enabled - there is a standalone reader beyond the goods gate adjacent to the gateline which spends most of each day covered up. Paper Travelcards do work the barriers however.
I also echo soil's sentiment about hearing how you got on.
... but when told about the NRG, he said that the maps 'weren't really used anymore',

He's not wrong there where EMT are concerned
Good to hear the OP has got the reservation. As others, I look forward to a post journey update.