Mahou Shoujo Miraculous Grausam appeared in Macht's memories back in chapter 89. He appears at some point later to erase Macht's memories of fighting the Southern Hero, which for obvious reasons we don't see.
Rivale and Tod are both mentioned in chapter 95, when Frieren is teaching Fern about killing demons.
Nobody has ever survived to report on Solitär, which means if the hero party encounters her they'll either choose not to report on her to preserve the timeline or get their memories wiped.
EDIT:
It is possible MSMG being a memory-wiper was a bluff, since they knew Frieren would see those memories. Assuming the identity of the memory-wiper wasn't already public knowledge.
More concretely, Rivale resembles the demon that murdered Stark's village in chapter 26.