It really does seem to be that easy! I was originally looking in the wrong place - in the data folder* rather than the db folder - but by deleting the Rupprecht line of the csv file and leaving it blank I find that the Rupprecht plan is no longer listed as a German option.
So, hopefully, the AI can't pick it again either!
Thus it would seem eminently feasible to force the game to run in historical mode by deleting all non-historical choices from all sides options. I just might do that.
* all the spreadsheets seem to be duplicated in the data folder and I have not altered these but it now appears the db folder data is what runs the game.