I'm no lawyer, BUT....
Here's a critical line from SWA's contract of carriage: "In the case of unchecked Baggage, the Carrier is liable only to the extent the damage resulted from its fault, or that of its servants or agents"... further defining unchecked baggage "(c) unchecked Baggage, including personal items, shall be considered to be the property of the Passenger in possession of the Baggage at the time of embarkation."
I think it would be pretty easy to argue in small claims that this was the fault of a servant or agent - most times the company doesn't show up nor did they send anybody. Just be wary in your locality as if you do it incorrectly or the company does show up, you can get burned pretty easily. If your lawyer is willing to help you out at a reasonable cost, probably not a bad idea.
And yes, welcome to FlyerTalk. I get that it wasn't intentional - but intent and fault are two very different things in the legal system. Based on the contract of carriage - I'd say you have a decent case. Pile on me or the OP... I don't care