So sorry this happened to you. I fly twice a week, on average, hundreds of flights in my life, and have never had an experience remotely like yours. I feel awful for you.
My one fear for you at this point is that the lawyer will cost you many times what the simple insurance copays will amount to. Southwest is a huge company and they will fight you tooth and nail rather than set precedent. I wasn't there and I can't say for sure if I think they were negligent or not, though I'm inclined to say no. To be honest, I wonder what kind of plane it was. Generally speaking, the angle of the overhead compartment is such that a falling object will drop directly into the aisle; they're designed that way, to prevent carnage if a plane hits turbulence, the overheads pop open, and items start falling. Was your husband in the aisle, or was he leaning over the arm of the seat at some kind of funny angle? If he got hit by a bag while seated, how did he fall to his knees? Does he have memory of this, or are you relying on the recollection of the 7YO? Because I'm struggling to visualize how this happened, I find it very difficult to say if SW might be negligent.
In any case, I'm assuming that with a history of spinal cysts and care by specialists, your family has decent medical coverage. I might pay the copay and let this one go. The indigestion, aggravation, and legal bills you might accrue in pursuing this one legally might add more stress to your family than the accident itself, and it sounds as though it's already been a difficult year. Wishing you guys all the best.