As you have pointed out Moridin can't simply read Rand's mind. Secondly he more or less told Rand how to kill off forsaken for good using balefire. My own personal take on this is that Moridin knows that the forsaken are past their sell by date and aside from Demandred to command the armies they have for the most part served their purpose and the more balefire is used with the pattern weak as it is the better for the Dark One.

Could Moridin have saved Graendal? Possibly. He has shown up before keeping tabs on Sammael and Grendal as the wanderer and he has every reason to keep an eye on Rand.

Rand did move kind of move suddenly and the problem the forsaken have is that what Rand did to destroy Graendal was more what they would have done not what something that either Rand or LTT would ever have done. It might not have expected him to be quite so ruthless.

Lastly I'm not certain if Moridin cares if Grendal lives or dies after she messed up the country. Her job was done and with her out of the way that one less backstabber he needs to keep an eye on.