Memory Replacement [Book 4]

Once Eragon and Saphira exit the Vault of Souls with all the dragons, Umaroth tells them that their memory would be erased to keep the eggs secret, but did Paolini ever mention if that memory was replaced by something else? Because the following chapter Eragon interacts with Umaroth as if nothing had changed, so Eragon remembers encountering the eldunari in the vault to some degree (at least supposing that the memory wasn't replaced).

But had Galby won, wouldn't he discover where Umaroth and the other dragons were hidden?? And if he knew, what would stop him from checking the Vault and discovering about the eggs by himself?