Maybe the best thing is to try a kernel compile, plus eventually, recompile busybox? Wouldn't this be typical of 32/64-bits linking/loading conflicts? (I don't know any details here, just know that ELF32/64 libraries can't always be mixed and matched.
Here's my 64-bits type info of the first file busybox isn't able to run:
On original Knoppix6.4.4, even if it runs with arch=x86_64, /bin/mount has: setuid ELF 32-bit LSB executable. Could the dynamic linking have any impact here?