I agree hanging a bag below the bars will hit the tire but give it a try using non stretchy straps. I don't see a problem with it rubbing the head tube, just put some cloth tape on the head tube. I wonder if aero bars would extend the load out too far and high, how about aero bars or bar ends that drop down and forward of the stem and lay your load lengthwise from the top of the stem onto the clips/aero bars. A place to rest your chin on descents but more importantly get the load behind the front wheel.

just thought of something, maybe I missed it but is anything mounting behind your seat? wrt the front gear break up those different items in such a way that you can put one item above the aero bars/bar ends and another below with straps holding them together and back against the stem. And if nothing is planned for the seat put the rest there. If camp pad is small it could go beneath with bag in compression sack on top and bivy behind seat?