Okay then secureinfo_a file must have been modified I guess, because I think maybe that is where the serial number is stored.. Maybe that is part of re-writing it with an other ctrnand like you did?.. I'm not sure..I read CW7253 from d9wip
I really don't know much about repairing 3DSs, so going to still have to wait for some one else to chime in that knows more about this stuff than I do..
If nothing else you can throw your 3DS in the trash can and buy a new-3DS XL or some thing, at least you will have a bigger screen and stuff.. But you would think that if you can still boot in to luma settings and decrypt9 etc that there would still be a way to save your 3DS some how.....just not sure how..
Although if you ever do get an other 3DS in your life, I might recommend backing up your NAND next time so you might be able to avoid a sticky situation such as this..