When asked if Semirhage was right about the DO betraying her, he said, "Semirhage is a very astute person." Which all but tells us that she got it right.

He also said at other timings that we don't have enough data to make assumptions about that scene, and any of the theories are possible.

Also, while your point that they couldn't expect Semirhage to torture Rand is possible, there was always the fact that Semi had to dispose of Min. That would have pushed Rand over the edge anyway, IMO.