Q. How to treat and repair root flare damaged by backhoe
I'm afraid there's no good news here. Damage to the root flare is serious because it can lead to trunk rot at a key point on the tree. All mechanical forces concentrate at this point on the tree so if decay occurs here then the tree is very susceptible to falling over. Not much can be done to correct this. I would recommend that you get in touch with a certified arborist for an evaluation.