I think booking the 1-hour connection at DFW with a wheelchair passenger was probably the biggest mistake (in addition to the double-connection; how much in hindsight would you have paid for SAN-LHR-TXL?). Your travel insurance should cover the cost of the overnight hotels. If you didn't have travel insurance, well, that seems somewhat foolish too given your situation.
AA was probably wrong to close the gate early (though Delta does this often as well), and certainly it sounds like you got a bad agent, although the fact that the police were called probably suggests his version of events would be somewhat different from yours. (Trying to be impartial here...).
After that it sounds mostly like a spate of bad luck; sorry to hear about it.