Is it permissible to resort to stratagem to recover previous messages from a girl for fear of them being used against him, along with the desire to repent and temporarily cut off the relationship with the intention of lawful marriage?

First, you must sincerely repent to Allah and sever your relationship with the girl. If you wish to marry her, then propose to her father. If he refuses, then seek another righteous woman. It is permissible for you to use trickery to obtain the messages from the girl, provided that no sharia-prohibited outcome results from it, or if lying is necessary to avoid a greater harm.