It can take weeks of separating before you can even take a leash walk with your adult dog and the new pup. Adult dogs hardly looooooove puppies that are not theirs by birth. it was your idea, not hers. Farming her out will make things worse as it doesn't teach her anything. You need to gradually expose her more and more to the pup. Don't forget the training and exercise for your older dog either. I have done this combo a few times with the pup always behind gates and in crate when the older dog was around off leash. Even a crate in a pen so that the older dog couldn't snarl at the crate door. These attacks you described could have very well traumatized your pup that can surface later in his first year.
So much of this ^. Over and over and over again.
I lucked out with my senior GSD. She loves puppies. It’s easier to introduce her to pups than older dogs. She takes right to the pups and mothers them. She’s had a ton of litters prior to me getting her, so maybe that has something to do with it. Who knows. Regardless, please listen to the advice you are being given very seriously.
You can damage the relationship your adult has with both you and your wife, you’ve allowed behavior that has resulted in injury to the pup, and farmed out the adult and let the pup stay. You need to get a grasp on your priorities, which should be adult GSD first, and pup second. You’ve likely set the pup up for fear of other dogs, and could make it worse by bringing the adult in and out from farm to home. You need a better system. Crate and rotate, baby gates, build a gate if needed. Do not allow the adult to see you lavish attention on the new pup. Make sure the adult’s routine stays as close as possible to her routine before you brought a puppy in. If anyone should be farmed out, it should be the puppy, not the adult.
I’ve attached a pic of the gates we build that are more user friendly than baby gates, and you can make it any height you need. You can get the pieces cut at Lowe’s or Home Depot, so the only thing you have to do is put the pieces together, and paint and install. This gate has self closing hinges so my little ones can’t accidentally leave it open. This one is near our front door, because we have a GSD/Husky mix that is an escape artist, and he would shoot past the kids as they were leaving for school. But we also have the same gates, just shorter, to section off areas where we can rotate dogs without having to crate them all the time. I’ll hang out in the family room with my senior and one of the pups, while DH is in the formal living room with the Husky and the other pup. Then we rotate pups, so they get supervised time to play and interact with the older dogs, but we are there with them to stop any bad behavior, which is normally the pup pestering the adult. When that starts happening, we let the two pups together and the two adults together. It’s much easier than it sounds, but we haven’t had any dog on dog injuries doing it this way.