I can eliminate the pinched tube between the tire question. The bulges are the root cause, sort of like the tube slipping out of the bead, but the bulge is inward, toward the hub. When I'm able to drop a photo, you'll see what looks like speed bumps on the exact inside diameter of the tube, running on the line of circumference, not perpendicular. One of the wheels I've built in the past was a double wall rim and it's the kind of bulge you'd expect to see if there wasn't a cap on the hole for the spoke, but these are ovals in the shape of the space between the valve stem and the adjacent spoke nipple. The first failure was also oval and fit in the space between the two nipples, and the blow-out was a longer slit.
http://www.dropbox_dot_com/s/txlg1oq...one.JPG?dl=raw
http://www.dropbox_dot_com/s/xizoxt9...two.JPG?dl=raw
http://www.dropbox_dot_com/s/6mlhncy...ree.JPG?dl=raw