Convertring my saltwater tank to african cichlids, few questions

boy does that sound familiar..... been there, done that!
I think your LR and sand have a huge bioload that is dying or has died. if you ran a reef you know about filtration, I would suggest not using the LR, at least at first, because of the large dead bioload. you could re-cure it in FW the same as you may have done initially in a garbage can with a heater and powerhead and tons of water changes.
as for the sand, I would fill the tank over the sand and add some bleach to kill everything (10%). stir it good, let it sit for a while then rinse and re rinse. to be sure you could use some de-chlor like Prime.
does your system have an emergency overflow that drains to your sewer? if so I think you could just run FW and let it overflow until the sand is well rinsed. the cold water would kill off most things left over from the reef, but I would still bleach it.

good luck, you'll love how much you save in electricity!!!!