Unlike with other Amigas, the A500(+) generates _KBRESET within the keyboard - thus if the keys Ctrl, AmigaL, AmigaR work by themselves, it must be this _KBRESET line that's broken and not the keys.
Start on pin 3 (pin 5 is keyed) and follow it back to the keyboard controller or to JP1/Q711 on the mobo and Gary's pin 5.