Great job, and thanks for posting!
To answer your question about the potential benefit of sewing the seams, I have done lots of destructive testing on the tube-to-tube seams used in the DIY Packrafts and the fabric around the weld always fails before the weld does – in other words the ironed seams are stronger than the fabric tubes – so while sewing the seams before welding might make the seams even stronger, it wouldn’t add any strength to the finished boat.
Regarding the leaks, I’m curious about whether or not you coated the inside of all of the seams with Seam Grip – if not, I definitely need to make this clearer in the instructions. Cheers!