Sorry about your emergency.
People will disagree, but you do not have to turn in your FMM. I have asked at a number of crossings and a number of different people (aduana and immigration) and all say it expires on it's own, no need to turn it in.
I can't say about today's forms, but there used to be an address on the back that the form could be mailed to if you have a need to turn it in.
Have you explored flying back and returning? If that's not an option, DRIVE VERY CAREFULLY!!! Stress makes us all tend to make bad decisions and drive too far too fast.