Can I stop & restart the cooking down process of apple butter if refrigerated in between overnight before starting the canning process?
It should be ok, except that I wouldn't can it straight from fridge. I'd stop cooking it now, before it's done, let it cool, refrigerate over night, and then finish cooking it before canning. Hope this is helpful!
It's a totally accepted process to cook your apples down into sauce first, then refrigerate and continue cooking down the next day. (This is when aromatics, etc, are added.) Food in Jars actually recommends to cook your sauce the first day, and reduce it into butter the next. Best of luck!