I thought the long period of the swell supporting the vessel at each end produced a bending moment breaking the vessel at the center point between the supports
And that could well be. They made educated guesses about the causes and there were signs of collapsed hatch covers in the wreckage. They determined it sunk whole, bow first, and broke in half when it hit bottom.
Your observations may be correct. I don’t believe they know for sure what happened. The hatch cover explanation is a good one.
The swells at the forward and aft ends of the ship could have produced that bending moment. Given that the ship was cut in half and lengthened some years before lends creditibilty to your point.
There is also another explanation that has been put forth-the Coast Guard or other agency went out and determined the various depths in the area where the vessel sank. There were a couple of shallow areas and with the swells, it is possible that the vessel scraped the bottom and tore a hole.
It is also possible those long swells caused her to bottom out over 6fathom shoals causing leaking. She was lower in the water when the same big wave that hit Andersen hit her.