Spells like modify memory can be undone with a greater restoration spell. What about memories literally stolen? The Raven Queen collects memories and other things and keeps them in her Fortress of Memories.
If she steals someone's memories, would it still be possible\reasonable for a Greater Restoration spell to work? If it did, would it literally pull the memory from the fortress, or would there then be two copies of the memory?
I understand that as the DM I can rule or set up the world however I like, and while I'm fine with taking creative license and switching things up I don't want to get too far away from how things generally work.
I will be using 5e mechanics in my campaign, but will be rewriting a decent amount of the lore as this is a custom setting. This is less about lore and more about mechanics, for the record.
Best Answer
Deities are a plot device, not a rules mechanic, so you decide
There are no published stats for the Raven Queen. She is a god, based on the quote for example in Mordenkainen's Tome of Foes, page 59:
In 5e none of the gods have stats. The magic of the gods is stronger than any mortal magic, as you can see for example from Antimagic Field
If anything, this would suggest that there is no way mortal magic can be stronger than a god's magic, and greater restoration would not work.
There also is this section in Mordenkainen's which suggests the way to get back memories that are in the Palace of Memories is to go adventuring in the Shadowfell. If you could just get them back with greater restoration, that would not be necessary.
There also is this tweet by Jeremy Crawford (not official rules but helpful to understand what the designers' thought) stating:
You write: "will be rewriting a decent amount of the lore as this is a custom setting. This is less about lore and more about mechanics".
Gods are a story device, not a rules mechanic. It does not matter if the rules suggest that the gods' magic is more powerful than spells the PCs have access to. In particular if you are building your own world, with its own lore, it is entirely up to you to define how this interaction might play out.