there is no question of downgrading here, just of not upgrading, which is quite different.
In iTunes you have the choice of upgrading or not.
Why should this choice disappear when you wish to restore your device to factory settings?
(After all, factory settings here were with iOS 5!)
If I try to restore the iPhone OFFLINE from the iOS5.1.1 that is downloaded in my computer, will it work?
Otherwise, I suppose the best issue would be delete all my content from the iPhone4S and syncing the data of my husband's older iPhone into the emptied iPhone4S, what do you think? (The only problem is this won't bring in the sms and call logs.)
Is there any third-party software that enables copying sms and call logs back from your computer to your iPhone? (I used to have Nokia cell phones before I got my iPhone, and I had a software that could do that.)
Then I'll be clearer. If you try to update or restore the iPhone, then iOS 6 will automatically get installed, so you cannot keep iOS 5 for long. And, you cannot install iOS 5 once upgraded to iOS 6. Note, that deleting everything will force a system reinstall that will result in iOS 6 being installed.