Nice way to put it together. If the coolant leak is the heater core I would think the coolant may be getting a wire harness in the dad wet which points to the computer causing the misfire nobody can find. Fix the heater core and see if that fixes both.
My YJ ecu harness is in the dash next to the heater assembly and heater core so you idea is spot on.
89YJ 2.5L TBI, AX-5, NP231 SYE, Ford 8.8, Yukon Grizzly, Tom Woods Shaft, 33" BFG T/As, 4" lift, Warn VR10000s.
AF Jeep Club Tail #184