Consider yet another option to solve the runaway trim problem. It appears that the most likely cause is a sticking switch on the stick. I like the option selected by Mark Kirchner on his LIV. Rather than have a rat's net of wiring and interconnect associated with relays that permit pilot priority over copilot, Mark put in a simple switch in the middle of the panel. Flip it left and the pilot has control. Flit it right, and the copilot has control.

Note that with this configuration if you get runaway trim, you merely switch the trim to the other stick and if the stuck switch is the cause (most likely) then at least the trim is now adjustable on the other stick. I chose this option which, as I recall requires a four pole single throw switch. I also put all the "need to get to them" breakers (like trim, hydraulic pump, door seal, autopilot and such) on the lower left side of the panel where they are immediately identifiable and can be pulled quickly. Other circuit breakers are hidden away but also easily accessible.