The busted lead has nothing to do with xmax. Woven leads have lots of extra slack beween the spider and terminals. Possibly bad solder joint, but looks more like they wire broke. The conductor likely overheated.
Depends how the sub fails. Sometimes they seize. An unpowered driver is more restrictive than a passive radiator.
When the first driver failed (assuming it didn't seize), it would have acted as a highly tuned passive radiator. The OP had 30hz jacked up 6 db at the amp and who knows what at the HU. There is a good chance that he exceed xmech since the sub would have unloaded at frequencies below whatever his "passive radiator" was tuned to and he was throwing alot clipped power at it at 30hz. I've never seen a tinsel lead fail because a sub exceeded xmax which is just generally 30+% of the coil outside the gap. Tinsel leads will fail when you exceed xmech, which is the mechanical limit of the driver.
I see no burn marks on the insulator to indicate over heating (at the solder joint). Even if the 1st driver seized, his enclosure size doubled when it did and he was throwing gobs of power at 30hz at the remaining sub. There is certainly a good chance he exceeded xmech. It could have been a bad solder joint, but the rest of the evidence available to us indicates this sub was pushed way beyond it's limits. It was seeing at least 2kw from the amp, plus the clipping due to the maxed out gain, plus 6db of bass boost, all inside an enclosure that is ~2x the recommended size sealed or tuned really high if the 1st driver didn't seize.