So I take it that once once on B8, there's no way to downgrade because the BL is upgraded and won't revert back. I flashed the older stock rom, and even Carbon Rom which broke wifi, with a "security pit warning: kernel." Guess there's a way to erase it.
I flashed back to B8 but something kernel-wise stuck and prevented wifi from coming on. I installed TWRP, formatted data and everything else a couple of times, then ODIN flashed B8 again. It worked. Assume there's no custom rom compatible with B8 that I can mess around with at present. Stock is boring and so Samsungish with tons of bloatware.
Anyone who gets stuck at the Sprint screen or the "preparing for first use" with the spinning circle, just rewipe and reflash (a few times if necessary). First notice of it working would be when it chimes at the Sprint screen.