but I wouldn't use the River Styx
Indeed. That's why I said I'd use the River Lethe.
My group has the 8-Ball now, and one of them is "infected" by its curse after asking it "how do we destroy you?" and getting the answer about the River Lethe (and a subsequent question to find out where the dang river was).
Now Step 1 of their plan is to get as close to the NeverNever River Lethe as they can, but the further they travel in the real world, the greater danger there is from human agents sent to retrieve the 8-Ball. If they just pick somewhere thematically close and jump into the NN sooner rather than later, they'll have a longer trek through the NN which has its own dangers. Either way, I plan for multiple shenanigans along the way.
When the 8-Ball is destroyed, I'm thinking that it keeps what it took and it takes back what it gave. Knowledge lost from Compels stays lost; knowledge granted from invoking the 8-Ball disappears (because they COULD ask for really game-breaky stuff before they drop the thing in the river and this is meant to counter that).
they have to make the decision to either never remember anything or to get every bad thing that has ever happened to them permanently etched in their memory..perfect and unforgettable
Remembering traumatic shit isn't a threat to my group of PCs. One of 'em has the Sight. One's a former Denarian host. One is fucked up from the war. The reason they take chainsaws to monsters now is because of the bad shit that they've seen. They need and possibly want those memories so they have that fuel to ensure such things don't keep happening to people.