I can still see conversations of people that deleted Facebook years ago and people that I’ve blocked even though their name comes up as Facebook user in some places.
If you ever had a direct chat with Imogen or whoever you’re trying to find (I’m a bit lost catching up) type in key words of something you know you had a chat about- say I don’t know, you discussed cars and you mentioned a mini- search mini on your Facebook messenger app.
You’ll see people but then you can also click see more under the tab where it says messages. Brings up all the messages you sent and received with that word in, might be a bit of a search but their name may pop up.
another thing you can try is searching her name on your messenger. I’ve a friend who deleted her Facebook around 2013 and her messenger as well but just by searching her name I can find her old ‘profile’. When I click on it, I can’t view her profile or contact her on messenger but I can see all our old conversations still. Hope this helps