For my money I'd use an Opto... somehow the measurement from -15 V to -9V seems wrong... but If the contact is more negative than the common an NPN opto, Emitter to the contact and collector to common would work. It would "Pull up" the contact to the common. Several of the Organs I worked on in the 70's used Pmos circuitry for the tone generator controls and they did have weird switching... PNP's in the voicing controls too. Most frequent complaint was dirty switches in little used devices. The switches on the keys were self wiping and usually Very reliable as they were the norm for the Pipe Organ consoles I worked on. There with the pipe organs the worst I ever had to deal with was a major mouse infestation. I just made the keys work for the Organ Tuner... he had to fix the damage to the air plenum... All Kid skin suede.
The droppings had to be cleaned up first... and a total Mess.
Bob