Hi Nickb,
mast break looks like most I have seen, in that it looks like compression failure. The damage to what I would call the back of the sail track does seem a little unusual though, usualy most of the compression failure seems to be on the side of the mast. Although only looking at pictures makes it difficult, the angle the back of the track is at could be from tearing after initial side wall failure.
When building Altered, I was asking about failures in carbon wing masts, I was advised compression of the side of mast was usual failure point, so I put timber web crossways inside, to stop it caving in. Seems to have made it bullet proof so far.
Spreader positions I have used is usualy halfway between diamond attachment points. Don't remember anything more technical than that. Top of diamonds is usualy determined by how close you can get to your hounds without fowling stays when rotating mast. Hound point is governed by class rules.
Regards Gary.