Laskern,
1. All the Rema says is that people plead at the graves. He does not say that they plead to the dead. The Mishna Berura (BTW, it’s s”k 28) says to speak directly to Hashem in the merit of the dead.
2. The Gra wrote to his wife and children when he was making his abortive trip to EY to his wife and children: “Be very careful not to go to the cemetery at all, for there the evil spirits attach themselves unto you, especially women. All the troubles and the sins stem from this.” This was affirmed by the Leshem and the Brisker minhag is not to go ever except at the funeral of one of the six closerelatives. See also Aruch HaShulchan YD 559:7 in the name of the Arizal (Shaar Ruach HaKodesh the block letter edition, p. 49) and Responsa Chatam Sofer YD 338, who also oppose cemetery visits in general and even calls them darchei Emori (superstitions rooted in a”z).