Judging by Google Maps and the scale on the map, the Armco roadside barrier extends about 25-30 metres back from the bridge. Doesn't sound far, but carrying or dragging a body would make it seem like much further.
Given that there was no evidence of mud, gravel, or plants on either the car or GBC's clothes, then EITHER it had all been washed off very thoroughly (those Forensic Crime Scene techs don't miss much - even a hair!) or he didn't go off the road.
I'm still thinking he stopped ON the bridge, perhaps where the bridge railing proper starts, and she went over there. She was lying in thick mud, and had left a distinct impression in the mud from laying there, so perhaps the mud softened her landing somewhat, especially if the tide WAS up high at the time. Perhaps she DID land with a splash in water - even if it was right near the edge of it - and that may have cushioned her fall as well?
But the problem with that theory is the plant material found in her hair, clothes, and body. If she'd been lying in the edge of the water, with tides rising up and down and either partially covering her or fully covering her (although not moving her according to the hydrologist), I'm surprised that plant material survived the 11 days without being washed away, especially in that ridiculously heavy rain we had just before she was found.
Another little mystery....