I'm glad the trainer is giving you some ideas for Amber. I am resistant to trainers or pet store employees giving me advice about their diet or nutrition, especially if they're recommending a switch to a grain-free food. Your pup is a
puppy and the concept of corn or any other ingredient in a well known, well researched dog food causing your dog to be hyper, I don't think there's research to back that up. Lots of breeders feed and recommend Purina Pro Plan and wouldn't do so if it caused issues. I would not change her diet if she's doing well on it otherwise. You also have to be careful with rope toys- if they pull out strings and swallow them, that can cause problems. I'm not saying my dogs don't ever play with rope toys but I don't allow them to chew or gnaw on them.
Are Rope Toys Safe for Dogs? | Animal Hospital | Germantown, MD
So, for those 2 things, I probably wouldn't continue with that trainer. If you and your husband are going to go it on your own, that doesn't rule out group obedience classes in your area where, for one thing, it can also be a relief to see that your own puppy's behavior is pretty much on a par with other pups the same age as yours.