Hi Pioneerschmioneer, your post has intrigued me a few days. It is confusing. Some scriptures are hard to put together. So I admit that I am not so good in explaining this topic. But I want to point out that the Passover lamb was very quickly prepared. It was basically killed without separating the head. And the guts were not taken out. So the preparation was quick, just kill the lamb and let out the blood, then put it whole on the fire.
So the Passover meal did not require much preparation. So I think the Preparation day was the Passover day for the next day that was allways a day of rest, the 15th day.
Exodus 12:8 ‘They must eat the meat on this night. They should roast it over the fire and eat it along with unleavened bread and bitter greens. 9 Do not eat any of it raw or boiled, cooked in water, but roast it over the fire, its head together with its shanks and its inner parts. 10 You must not save any of it until morning, but any of it left over until morning you should burn with fire.