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I suspect they lacked something in the quality control, rather than basic design, and the device flew apart faster than the reaction could proceed. Maybe the implosion was insufficently spherical. Or cylindrical if they were attempting that design (It's easier , but less efficient)

Again, we just don't know - a small, but effective neutron weapon of low explosive output would be consistent with the seismic data. I would assume this were the case. A neutron weapon apparently requires a certain amount of high-purity tritium; has NK been after that material?

Again, we just don't know - a small, but effective neutron weapon of low explosive output would be consistent with the seismic data. I would assume this were the case.

A neutron bomb is a very small hydrogen bomb that's highly optimized to produce a big neutron flux. Given that we have no evidence that the NKs have ever developed a hydrogen bomb at all, I think an NK neutron bomb is highly unlikely, unless they stole it or bought it.

Remember that the US didn't develop neutron bombs until the 1970's. It's a 4th or 5th generation device, not a trivial piece of technology at all.

The much more likely explanation for a 500 ton yield is either (a) a barely-fueled "proof-of-concept" fission bomb, with a possible full yield test later; or (b) a lousy design.