You know, a heavy dose of luck potion would almost justify messing with time. I still don't see any way to fake a failed ghost creation without transfiguring a witch or wizard. (Though in principle, Imperio would remove the need for a volunteer.) A few hours does not seem like enough time to grok ghosts in fullness and produce a guaranteed failure. Nor do you want any uncertainty. Never let under-determined Outcome Pump Physics decide if your plan will really preserve the appearances most efficiently, or if the Universe should create Uplifted utahraptors to stop you.