Wow that's alot of work! well done. Natural strings are even more rare than selfbows.
I'd check the small section in orange as it seems one of the ply is stiffer making the other twist around it.
It usually ends in one part taking all the wheight.
I'd also add some oil or wax as the dry vegetable fibers tend to become brittle
please let us know how it works
Lately i've made a few baskets out of elm and arundo donax canes
Elm bark is very durable and flexible and I guess it could be turned in a nice string