[knightofalbion]

Wondering: I'm very sorry to hear of your loss. My heart goes out to you.
Having to have your beloved pet put down is always a heartbreaking decision to have to make. You loved her dearly and didn't want to lose her, but she was suffering and it was a greater act of love on your part to let her go...

She is at peace now and yes, I certainly do believe that she is there in spirit and I also certainly believe that she appeared to you in a dream vision. Communication in the spirit world is through thought transference and such as this is a common way for those who have passed over to get 'messages' to loved ones left behind.
As you say, the message was that she is still 'alive', free of pain and happy. And awaiting the day you'll be reunited.
As for the mysterious person holding the leash. I suspect this was a subliminal message to show that someone was caring for her on the other side. [Usually pets are cared for by a loved one of the owner until their 'time' comes. If not that, then there are those in spirit, who have a great love for animals, who dedicate themselves to caring for pets until such time as their owners pass over and owner and pet are reunited.]

[I use the term 'owner' advisedly. It is a bond, not ownership. But that is the term generally used and accepted.]