Can species back-evolve?
Complexity is a lure. Evolution isn't defined by complexity, but by adaptation to one's environment, as you mentioned.

Evolution is an ongoing, non-deterministic process. There's not one definite evolutionary "path" that brings a unicellular to a modern human being. Nor is there a predictable evolutionary path that brings a dog into becoming a human. You also have to acknowledge that, because evolution is ongoing, the dog of yesterday is not the dog of tomorrow. That applies to all the living.

Is it possible that given the right amount of time and the ideal environment the modern human could evolve into the modern dog?

Yes, it is possible.

Is it probable? No.

The chances are that if humanity were to find itself in such an exotic environment - to which dogs are 100% adapted, but humans 0%, which just shows how unprobable such an environment is - it would become extinct before it has a chance to adapt, much like a lot of other species have done.