IOS – Does iOS 5 have a SSL security bug that got patched on iOS 6/7

Sadly, no.

The iOS 6.1.6 patch (as was 6.1.5) is an update only available for iPhone 3GS and the fourth generation iPod touch. Apple appears to only be maintaining that branch exclusively for devices which were never compatible with iOS 7.

The list of trusted roots in Mavericks can be found here: http://support.apple.com/kb/HT6005

However, I'm not actually aware of a way to modify the "System Roots" keychain, so any new certificate authorities installed would appear in either the System or Login keychains.