I heard a rumor (from several independent sources) which indicated that the firing sequences are essentially encrypted detonator timings that are passed through the PAL, which decrypts it but makes no value judgement about the timings themselves. If the timings are wrong, you get a messy squib explosion which will make a mess for about 100m around the detonation site, and which will totally destroy the weapon beyond any hope of recovery.

One would expect that there should be something in the permissive action link that prevents a radioactive mess as well, but I have heard these rumors too. I always wondered if you could do dial a yield this way. What you are inferring is a bunch of cables of different length (delay) or the equivalent between the firing circuit and the detonators. The input delay information would specify which delayed version of the detonate signal goes down which path. I get the impression that PAL is a little more complex than that, one of the reputed goals is to prevent a weapon from being easily modified to go around safeguards. Playing with just delays can be overcome by characterizing delays in a dismantled weapon.