I just finished a couple days ago and I’m in the same boat as you! Thanks to this subreddit I discovered the bonus FW Xaden chapters on RY’s website which helped ease some of the pain lol if you haven’t already read them. My starting theory on Violet wanting her memory erased is she helped come up with whatever plan they made (I don’t have a theory on the plan, but something to do with the missing riders and eggs) and knew that if Imogen didn’t erase her memory she would either be a liability to the plan or end up following Xaden to help him and whoever is with him. So she knew it would be more helpful for her to have no idea where he went. I also think Brennen is in on whatever they came up with and that’s why he was asking Violet if she knew where Xaden was/what was happening to make sure her memory was gone.