I would not remove any more than you have too - expecially with an out of production BC.
The cable almost always attaches to the inflator with a simple pin through the cable loop. Just use the old cable to the dump valve. In the event the old loop is too small and/or in the event you add a longer corrogated hose and consequently need a longer dump cable, cut the loop off and using 2 cable crimps crimp a longer piece of cable on and form a new loop with another crimp. I woudl not mess with the shoudler dump at all. Many plastics do not age all that well and while srong enough as is, something may crack or break if you try to disssemble and reassemble it.
Plan B is often to replace the entire shoulder dump and corrogated hose aseembly - but that requires the hole in the BC to be a standard size.