How do I recover an entire conversation in Messages on iPhone and Mac after accidental deletion?

My wife accidentally deleted an important conversation in messages on her iPhone. When she recovered the messages only messages for the past month were recovered. The conversation stream on her Mac mini is the same, just the previous month. I signed out of iCloud, restarted the devices etc. but the same conversation stream(past month only) is still present. However I checked her MacBook Pro, which she hadn't used recently, and it still has the entire conversation stream, not just the past month. If I push sync now on the MacBook will the messages be restored on the other devices, or will all the old messages be deleted from the MacBook. All the devices are signed in to iCloud with the same Apple ID.

[Re-Titled by Moderator]

Original Title: Incomplete Messages