Extending mast head fitting rearwards to allow wider sail to clear back stay.

Running backstays...

You do seem to get to a scale point where relying on leech loads to support the top mast no longer works. On boats like the 12s where the main is never squared off because you are always sailing hot angles this is a lot greater than it would be on a slower boat - I think if you were to sail a 12 dead downwind with the boom squared off on the shroud the mast would be very much at risk. As doing so is slow the problem doesn't arise.

In the Thames Raters in the UK, which have extraordinarily tall rigs and must run square (can't really gybe downwind in a river that's 6 boat lengths wide) the conventional setup appears to me to be for the centre hand (they are three handers) to be in charge of the running backstays and nothing else! Obviously its gybing that's the issue, tacking is not a problem.