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Is there anything wrong with rappelling on this? Using a 9.5mm single rope on the rappel and ~5mm tagline. Overhand on a bight knot for the 9.5mm that the carabiner goes through then joining the 5mm accessory cord tagline to the 9.5mm rope with an EDK. In this situation the 5mm (and EDK) isn't taking any load besides pulling the rope down.
As far as I see it, the only danger is the EDK knot getting stuck while pulling it down on a multi-pitch climb.