Downgrade to iOS5 from iOS 6.0 with iPhone 3GS

After upgrading to iOS 6.0 5 days ago, I've discovered that lots of files have been missing (photos, contacts, apps...).

And trying to restore to previous version iOS 5.1.1 IPSWis not validated by iTunes.

I could use my backup on Time Machine, but last update having been made 2 weeks ago,

I don't really want to do it and lose one week between backups.

Any ideas?