The channel on the ends only has to be big enough for the cord. I just eyeballed it.
Through the channels I used some bulky 3mm nylon cord from REI that I had lying around.
If anything, your pitch might be a little too steep. The rule of thumb seems to be 30-35 degrees and you are looking closer to 45 degrees. Those anchor points are very close together. This might be part of the reason you feel a bit cocooned, but some shoulder squeeze and cocooning is normal. At the steep pitch you currently have a Ridgeline won't do a thing for you, but as your anchor points get father apart I think a Ridgeline is a must. It just makes setup so much easier and longer spans more usable.
I also use ripstop from Joanne's and, Hancocks. Maybe you got a bad few yards? I've got the equivalent of a few months in one of my DIY hammocks with no signs of stress.
There would be no function difference between taffeta and ripstop. Many commercial builders use taffeta which is, IMO a superior fabric for a hammock unless you are a gram weenie. Ripstop, as mentioned above, was designed for clothing, not weight bearing. The ripstop grid is not really a major benefit in hammocks.