Are the gene defects causing things like heart problems common to particular dog breeds such as Cavalier King Charles spaniels, healable using a Lightworker Healing Protocol session?

This, in fact, can be done, and often done readily. Even though this is a genetic defect and will more than likely manifest left on its own, it can be healed when it does and it can be prevented before symptoms occur by doing healing work for the animal with the Protocol because it does request repair and healing for all genetic distortion. This is a beautiful illustration, in fact, of the reach of the Protocol, to go out to another species and deal with an issue that is seemingly hopeless at the present time to forestall, and whose effects are tragic, causing great suffering not only for the animal but especially so for the owners who spend time dreading its appearance and then fretting over their beloved pet over months to years, watching them suffer and decline in function only to die young in some cases. Having a way to prevent this is a godsend and that indeed is the point of all divine healing.