If the original person effectively assigns 0 or 1 "non-updateable probability" to some belief, or honestly doesn't believe in objective reality, or believes in "subjective truth" of some kind, CEV is not necessarily going to "cure" them of it - especially not by force.

I think you're skipping between levels hereabouts. CEV, the theoretical construct, might consider people so modified, even if a FAI based on CEV would not modify them. CEV is our values if we were better, but does not necessitate us actually getting better.