Variables
Hi and welcome back,
First, the "1.5" you mention is not a safety factor -- that comes later in the calcualations -- but the typical multiple of the load at 30 degrees of heel to the maximum load, typically at around 60 degrees. Your calculations' result is correct for the mainmast, in any event, so yes, the 9,000-ish lb load on the uppers is likely right, or right enough to select wire size.
No idea how you and the rigger could be off by a sixteenth, but this is something I hope you can clear up; if those uppers are really 1/4" they are probably too small. Any chance they might be 9/32"? This diameter would be better than 5/16" for the above numbers.
As for those jibstays, I am always wary of a "them're like that" argument, and this is no exception. It is indeed very difficult to get adequate tension up there, and if you keep the twin stays you'll want to come up at least one size on your backstay, even if it forms a wider angle to the mast than the jibstays. But it should be possible to prevent excessive sag on those stays with proper tuning, since as the loaded stay loads, it stretches the backstay, allowing the unloaded stay to shrink, casting off some of its load. But even then, since you can expect at least 4" of sag on the loaded stay, and since the unloaded stay is probably less than 4" away, you can be heir to the delights of hanks jumping to the other wire, among other things. So it doesn't really matter how well this rig can be tuned; switch to a single jibstay configuration. You'll have less weight aloft, less windage, less compression on the mast, a smaller backstay, less load on the hull, and less bother altogether. That's why this configuration was abandoned long ago, along with wire reel winches, twin running poles, and other technological experiments.
As for the mizzen, you scale it, not with a smaller safety factor, but with a smaller load factor, since, with few exceptions, there simply isn't enough sail area in the mizzen to generate anything like the maximum vessel righting moment with that sail, no matter how hard or unexpectedly the wind blows.
Fair leads,
Brion Toss
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