From that pic, the 7550 elbow is a better, sturdier engineering design believe it or not. However, the strain relief on the 600 is better as it is longer and can dissipate stress over a larger area but also worse in that it's actually two pieces. So the cable actually gains no support from the surrounding material against yanking. The 1000 is a bad two piece design IMO and puts all the stress at the wrong point if you ask me. Maybe there is more going on inside than the eye can see. The plug is pretty though.
You sir, are spot-on. Some users of the EX1000 in China report rapid (some in as little as two weeks) plug failure. And yes, the EX600 cable gets no actual bracing inside the relief tube.
You deserve cake. Too bad I had the stove on "Incinerate Test Subject" and not "Bake Cake".