Again, once I complain about being stuck I manage to unstick myself - using the exact same strategy, only this time it worked. There's enough tiny little random steps along the way that the same battleplan might go completely differently if the dominoes line up. I am now past Belberith and therefore ALMOST DONE.
... with a first ending. out of i don't know how many.
... Although there is a NewGamePlus mode so it should be easier to go through on play two at least. How eager I am to try out other endings will probably depend on how interesting this one is.