Thanks for the explanations on EUCLASE and NOUP. I didn't know the former, but do now remember seeing the latter.
I did wonder whether the setter could have asked the solver to distinguish between the 25 filled cells remaining in the thematic area. The solver is likely to need to check on the history of the game anyway.
Perhaps that would have given rise to confusion seeing that half the cells also needed to be shaded. Even so, it was a good puzzle, and indirectly introduced me to the word 'zugzwang.'
Season's greetings to all contributors.