"Based on my knowledge of seismic detection and verification of underground nuclear tests, they could not have detonated a device without it being discovered. You cannot decouple it successfully, and the signature is very differnet than a chemical explosion."
What if they had a nuclear accident before the bomb was tested during the assembly process or an earlier process? Could a nuclear accident caused these problems? Then, they would have had to go to their cover story.
If it produced yield, even a few hundred tons, not even 1kt, it would have been detected. There could have been a criticality accident that sprayed material around, that would have caused ugly problems like Chelyabinsk 40.