I think you are going to be severally limited by your space. As you can see from your shots, your are not able to get all of the trees or all of the dog in the same shot. You might get by if you are able to get smaller trees.
If I have pet shoot, I try to get at least a little of the background in the shot. In your above shots, the tree tops are cut off and even the paws at the same time.
The issue you will have as you mentioned is your 85 is WAY to long for that space, and the 18-55 is going to give you some serious distortion issues if you go wider to get everything in the shot.
It is almost a case of needing something like a garage to get your shots set up the way you would like.