Hi Nick,
Just looking at your mast-break reminded me of mine a few years ago, going upwind in about 18-20knots with 170kg on the wire (fat friend
), the mast rotator arm bent suddenly allowing the mast to over-rotate rapidly, and we hit the water with a new two-piece mast. It broke almost in the same place as yours, a little closer to the spreaders in my case. My theory is that the mast rotated to 90deg or even past, with 2 on the wire the spreaders are facing fore-and-aft, no lateral support at all, bang. The other factors you mentioned could also be the cause, I`m just offering a possible explanation if you had not set the mast rotator yet and it was over-rotated.
I don`t believe the squaretop caused the failure, but the additional power may have assisted, although I`m not convinced as you say you had not powered up the main fully yet. There are too many possibilities, only way to know for sure is to get a new mast and try it out again ! (just insure the mast first
)
Steve