I actually have both a disabled dog and a masters in Design, so I feel well suited to comment on your work. My dog has lost most of the use of his rear legs due to a neurological disorder. He can drag himself around, but cannot run or walk any distance. He can't get up stairs alone, either (and I live in a three-story walkup). My dog will never get better and will need help for the rest of his life, so an aid like this is essential.
This project, as with all ID projects, could be greatly helped by focusing on the end user, not experts. Partnering with users and trying to make a simple mockup can tell you so much about how to improve a concept.
One HUGE problem with your solution is going to the bathroom. The dog would urinate all over your device, making it very unpleasant to use more than once.
http://www.walkaboutharnesses.com/ makes a very simple fabric sling that serves the purpose of aiding my dog to walk and get up and down stairs very well. It has cut-outs to allow him to go the bathroom with some help. It cost $60.
I also bought a wheelchair on eBay made out of PVC tubing for $100 with shipping. It was made by a very nice lady in Buffalo, NY.
This concept has some potential that could realized with further research and development.