I sure hope that FR would cover this under warranty, including the damage to the canvas, but I hate to say, don't hold your breath.
I had a roof collapse on my second or third camping trip due to a shattered gear in the powerwinch. They did reimburse me for the new power winch, but they refused to reimburse for damage to the canvas or a gouge in the interior wood (that was probably caused by workers when repairing the trailer. In other words, they quickly reimbursed me for the "actual failure" but any collateral damage was ignored cause I could not prove it happened due to the failure or was damaged caused by previous openings and closing (which were just two or three).
As for your situation it appears the spring kept coming down but the roof got stuck on something, in my mind, knowing how the goshen system works (pretty well), that is the only way I could think that the spring would be able to pop out of the lift arm.
Mine when raising, does make popping noises at the lift arms, but I always attributed the pops to the inside lift arms extending out of the other lift arm (I believe there are three per side). And being around other similar popups, they all seem to make the same popping noise (and believe me, mine are very well lubricated). But it is just a popping noise, so hard to tell if your noise was louder.
As for the cable, not sure how you "push the cable." Did you place our foot on the cable under the tongue and push? Mine, the old cable as well as the new cable, always traveled nice an easy onto and off the winch drum. The tension on the springs, caused by the roof, are what push on the cable to lower the roof (one reason why it takes a lot more energy to raise than lower). Once in (approx.) 10 times lowering the roof, one corner might hangup with about 12 inches to go to close (but only a few inches different from other corners), and what I would do is go to that corner and push down and it would pop down.
Sitting here, and not seeing it in person, it sounds to me that the roof was hanging up on the lift arms (or something else) allowing the springs to move downward, being pulled by the cable and then suddenly the roof became free, fell, and the spring popped out of the seam between the lift arms (on that side of the trailer). (Need to ask, did you ever lube the cable tracks under the trailer? Almost sounds like the cable could have been hanging up in the track due to not enough lube.)
I would think that if it is still under warranty, the dealer needs to fix it and make it right. You could also try a different dealer, but then you would have to fight with FR for reimbursement, since FR dealers usually do not perform warranty work on units they did not sell. They are not like the auto industry.
So, if it were me, I would take it back to the dealer, call FR customer relations (if they have one) and get them to repair the problem.
Good luck, please keep us informed of how you are helped (by dealer and FR); especially for those who are still under warranty.