You dismissed the most important piece: the first place to start is the redirects. It will take work, but probably not nearly as much as you think. It can be done in a more dynamic fashion assuming the data used for the URLs was pulled from the database. I just helped a website with over 2 million URLs of many different formats create these redirects. In this case it was ahead of time, but we can map it out together if you need help. I know this is time sensitive and no one has responded with an answer yet, so if you need a call ASAP let me know and I'll try to flex around your time. We'll also briefly cover other strategies you should implement to get back in good stead with Google.