the rig snapped....several things are working against it. First, the main is WAY eased. Second, there's not nearly enough vang. Both of these issues are allowing the main to leverage against the spreader (big main too) and the combination of twist and column distortion further exacerbated by stuffing the boat in to the chop has made a tough job for this poor tin rig.

Mutiple factors are in play including lower vs upper tension

it's distortion is centered 2' above the gooseneck and the main has the wrinkles of death
They determined that the backstay didn't prevent the out-of-column issue happening 2' aboave the gooseneck whereas the checkstays did (U20 big main)

Adding Dyform also helps as the 1x19 gets the shit stretched out of it when loaded and the dyform just holds it's length MUCH better.

Blowing the vang dead downwind with the main eased out like that is the exact opposite of what you want to do. Essentially it does nothing to depower the main, but instead shoots the center of effort up into the top 2/3rd of the rig. The leech of the main is your backstay!, so with the main eased all the way out and vang off, you basically just blew your backstay off.

You need traveler down and sheet on to keep at least some pressure on the leech of the main, otherwise, rigs gonna come down as well. Without the main, you only have the caps to save you and that creates huge compression loads in the rig.