Like I said, the trees I hang from are no less than 6" and no more than 12". Yes, there are places I go where trees are bigger or smaller but I don't hang there. Also, if I ever did go someplace where trees were bigger, it wouldn't take much to include some longer tumplines.
As for the orientation of the fibers, you are absolutely right; using fabric "tumplines" is inefficient because the load is being carried by only half the fibers. I would like to find some ultra-light Spectra or polyester webbing 6" wide and 0.001" thick but I haven't so far, so I will play with what I do have - which is still in transit as I type this. I have considered using some kevlar or spectra tape from the composites industry but I don't think it would hold up well since it is meant to be used in a resin matrix.
And yes, I am making this MUCH more complicated than it has to be.....but its FUN to consider the possibilities! HYOH.