Guided Rappel at Cascade Creek
From Tom's rave at Ouray: http://www.canyoneeringusa.com/rave/...ugust-16-2012/
From the report, it sounds like you have the contingency plans covered with a good outline of the pros and cons in your situation a) jugging b) lower from the top c) relaxing the guideline tension. Maybe if Kevin had a long sling, webbing, or pull cord, perhaps he could have attached that to the taught guide line (replacing the original) and slowly lowered himself to the waterfall, unhooked from the guide line, then continued as a normal rappel.
It wasn't clear what you meant about "As a result of the geometry at the on-rappel spot, the rappel line crossed over the guide line below the rappeller." But, we all know, ropes tend to get crossed quite a bit. However, if there were people below the rap, wasn't it possible to make sure the ropes were separated before the rappel?
Thoughts?