I am not going to speculate on Satan's motives and schemes.
You probably should if you're going to be a Christian. He's just as real as God is and he's constantly at work trying to lead people away from God. That's his motive and he'll use anything and anyone he can to do so.
So, think about what you read about the Blessed Virgin Mary in Maria's writings. Why would Satan speak about Her in such a way, when he hated Her Son to the point of killing Him, and so how much more would Satan hate His Mother for bringing Him into the world to redeem mankind and defeat him?
And, Satan wants to lead people away from God, and the Blessed Virgin Mary wants to help lead people to God, and thus, wouldn't it be more befitting of Satan to either downplay or deny Who She is, what She's done and does, and the degree of influence that She's had and has, and so on, and get people to believe that, in order to lead them away from God? Because for Satan to speak of the Blessed Virgin Mary in ways that honor Her and draws people to Her Son, and thus closer to God, is the exact opposite effect of what he wants. Do you agree?
And, we agree that Maria Valtorta's writings have a supernatural origin, but you think it's Satan's work not God's, and that he's saying lies about the Blessed Virgin Mary. I take it that you've ruled out the possibility that people's interpretation of certain scriptural verses are wrong, and that what Maria Valtorta was told and shown is actually the Truth? How did you come to that rule out?