How dependent are backups on msdb tables like backupset?
If you have a single file with all of a set of backups in that single file, you should be able to use the command:  

 RESTORE HEADER ONLY FROM backup device ...


This will provide you all of the details within that file, including the log file LSNs and other details you might need.

http://technet.microsoft.com/en-us/library/ms178536.aspx

The EXPIREDATE and the RETAINDAYS are just data that you include with the backup.  You have to decide how to use that information.  In an appended file, it is mystifying indeed. You could perhaps use the information to skip over backups that expired.  (For me, each backup is a separate file.) See Tibor Karaszi's comments at:

http://sqlblog.com/blogs/tibor_karaszi/archive/2012/07/08/backup-and-the-evil-retaindays-option.aspx