What do I do with the toy if my dog does not want to carry his handle or toy on our walk?
You'll love this part. If at times your dog chooses not to hold the toy, the two lines simply snap together creating a “regular” leash. You can unclip the toy and we have provided a carabiner for convenience near the human handle where the toy or handle can hang.
I noticed the dogs you feature were retriever breeds, how difficult will it be to use with non-retriever breeds?
You are right about the retrievers on video but we've had great success with other breeds as well. Basically if your dog will hold anything in his or her mouth at all (including a rawhide) it should be easy to encourage him/her to hold the leash. Any necessary effort is well worth the advantages-for both of you.