Hypothesis 3

Bone fracture with nerve damage.

The surrounding nerves are at least partially intact as evidenced by the deep palpation pain sensation and the intact motor function.  Also, the reflexes are inconsistent and non-repetitive in the hind legs.  This could be suggestive of nerve damage too.  The fact that the dog is refusing to stand could be due to damage on the innervations to the support muscles.  The nerves could have been damaged during the blunt trauma from the accident or directly from the fractured bones impinging on them.