iPhone restore error "This device isn't eligible for the requested build" - tried everything..?
Your problems are entirely on Apple's end. The message indicates that Apple no longer signing that version of software.

Yesterday, the code signing servers would sign the iOS 6 package you had downloaded on your Mac and let the device install that version of the OS.

Today, Apple is no longer signing that version of the software for that device.

I don't know how to say this other than Apple is forcing your hand to upgrade whether you are ready or not.

Had you used a jailbreaking tool to save the signature, you would have been able to replay that signing activity and possibly trick iTunes into restoring iOS 6. I wish I had better news, but you may have to jump to iOS 7 unless someone invents a new exploit to jailbreak iOS 6 in the absence of Apple's signing servers being cooperative with iOS 6.