Your question is difficult to answer because I don't understand what you are try do. First of all if you do exactly what you stated it will not be a true 1:1 copy because DVD Decrypter has at the very minimum removed the CSS (encryption) protection and maybe the region control and macrovision too. But did you really mean by "a true 1:1 copy" to retain those "features?" Probably not. There is also a question, already asked, why are you using both DVD Decrypter and DVD Shrink? If you want an "almost true 1:1 copy" DVD Decrypter alone will do it, just use the ISO mode and the turn around and burn the ISO image, if there is space for the burn. This will give you an "almost true 1:1 copy." If the image is too large to fit on the disc you are using, then it must be "shrunk" somehow to fit on the disc and something must be sacrificed. DVD Shrink is one of many tools which will help you do that. If you wish to post more information on what you are attempting, there is probably help around, in addition to the many guides.
Your second post:
"Does DVD Shrink decrypt all discs?": I don't know, but probably. Try it.
"Is there a degradation in quality?": There is a degradation in quality anytime you reduce the compression or "Shrink."
"If I use DVD Decrypter and the DVD is 4.7gb or under, then I can just use Nero to burn back to a blank DVD. I would not need any other program at that point? Decrypter and Nero only?": I think you can use just DVD Decrypter alone, but I use both.