Here is what we do, because we have the same problem. Technically, it goes against the directions, but I personally have no problem making the decision to let my child do this, because I understand the crash mechanics andother iossues involved, and do not believe I am sacrificing safety at all by doing so. While I can not recommend you do the same, I can inform and educate you on the issue, and let you make your own informed choice.
We push the chst clip down several inches. When you do this, it allows the straps to be wider apart, and not cut into the neck as much. And here is why I do not believe it is a safety issue. The chest clip is a pre-crash positioner....it's function is to make sure the straps are correctly positioned at the time of impact so that they may function correctly. A child in a properly use, properly tightened harness, does not even NEED a chest clip, and for example European seats don't even HAVE them. (Just us dumb Americans who cant be trusted to tighten our straps properly...) In the case of the nautilus, when the chest clip is placed at chest level, it causes the straps to be TOO clase together, pulling them in unaturally, as in the pic you provided. Pushing it down a bit allows the straps to fit BETTER over my child, straight down over her shoulders, nice and snug, wityhout being pulled in TOO close to her neck. The straps are in the correct slots, and are properly tightened and buckled, and therefore the chest clip being too low does not reduce the safety at all. In fact, I actually have some belief(although no proof) that it might be safER, because when i push it down, it allows the straps to sit flat, in a straight, tight line, and when it is up higher, exactly like in your pic, it actually curves in..which, in a crash, once the chest clip breaks, that curve in the strap will actually be just extra slack.
so anyway...that is my long-winded explanation of why the chest clip inmy dd's nauti sits just above her belly button (about 2 inches lower than chest level).
I know you said it didn't help, but are you sure? could you try again? and uh...car seat tech was..not exactly wrong...but simply repeating rote facts, without taking into accuont the reasons behind why we do what we do. *shrug* I can think for myself, i'm not an automaton.