Backstay with sheave and 7x9 wire OK?

[Brion Toss]

Hello,
Sounds like your backstay is split for two reasons: to accomodate the rudder and tiller; and to provide a 2:1 purchase on the pendant. I agree with Ian that a block can be a vulnerable item to hang a backstay on, particularly with a masthead rig, but will note that an adequately stout block can be utterly trustworthy. Still I'd likely only use one with a cascading purchase at the bottom, rather than a turnbuckle, because to me a block's big advantage is range of motion, and a turnbuckle doesn't readily provide that.
That's why we would, instead, install a linkplate down as low as possible, while retaining a suitably narrow angle between the legs. Then we'd tune the backstay such that, in light airs, the delta plate was off-center, putting most of the load on one leg, and that in heavy airs the delta was on-center, with the legs equally sharing the load. We'd put a long-throw adjuster on one leg, and a regular turnbuckle on the other. That way you could establish and adjust basic tune.
At least the turnbuckled leg would be heavier than usual, just in case a gust hit while the stay was in light-air mode, and a lot of load got onto that side.
In any event, I see no logic in splitting the backstay up near the masthead. Perhaps I am missing some piece of your rigger's logic here.
Fair leads,
Brion Toss