hi there FtW, been a while since I've seen you poke your head up here. Hope the Oregon thing is working out for you and yours.
I'm wondering if in your application of the whoopie sling, the constrictor and the bury are pulled a lot apart, e.g. if where you attach on your stand has a significant diameter and you have to cinch up tight. My guess is that most of us doing the whoopie thing have those lines quite parallel where the bury emerges from the constrictor. This seems also to be true in the pictures I've seen of the sling used in arborist applications.
My intuition, for what's that worth 'cause I sure don't have a large body of experience here, is that you don't need to worry about failure. The rope isn't making a tight turn, you've got all the rest of the bury and constrictor working for you, and I don't see that you'll get much more unraveling if the cinching up you do isn't getting any tighter...the stand isn't shrinking.
but then maybe someone who knows what they are talking about will chime in...