The threaded rod is to be placed inside the U-channel, like this:
I was originally gonna use that bit there as an adjusting nut at the head of the neck, but now i decided to just have adjusting at the heel.
As you tighten the rod inside the alluminium channel, it tends to warp along the "open" side of it, which is to be placed facing the back of the neck. Since it will be limited on its sides by the wood on the neck, it will counter the tension on the strings if you tighten it.
Here's the truss rod very tightened in comparison to a straight edge:
AS for the weight of the body, it will indeed be very light. I have never used this wood, but it is very traditional among luthiers here. About that small tearing i had, i spoke to a luthier friend of mine and he told me it was very common when using this wood. This is why they use it mostly for solid colors: so the wood can be filled underneath...
Cheers
André Ripoll