If what you say is correct (booting OFW, not even through hekate), doesn't boot without sd card), that's not good.
That probably means that's something wrong with your sd card reader or with the sdmmc1 clocks, so basically a hardware failure.
I can't help you today, but tomorrow I can send you a debug version and check where exactly hangs.
On the other hand, I don't believe that this will help you with Horizon though (I hope I'm wrong!), because its sdmmc1 init is similar (but more complete) with hekate.
By the way, sx uses proprietary sdmmc1 and dram init/training, taken from google/nvidia, so that's why it works. Try to boot into horizon with this (so we can know that your eMMC is ok at least).