Hi Deb, What a frustrating time you must be having. I wonder if your design could be tweaked to utilize a thinner sheet of poly clay. It sounds like you've been doing a lot of the "right " things, but curing PC an inch thick is tricky, even under the best circumstances.
Sculpey III is one of the weakest of clays and even more prone to cracking than the stronger brands; PREMO (my favorite), Kato, Fimo. I have not had a huge amount of success curing things over 1/4", though it's possible and I have. I would like to suggest using the staircase/ramp method of curing. Use an external thermometer or one other than the one that is part of your oven to double check temperature before you start (oven's built in thermometers are often VERY unreliable). I can send directions on the best kind of external thermometers and how to run a test for temperature if you need them.
The following directions are for a PC brand that recommends curing at 275F. for 30 minutes. Start with your PC in a cold oven and cure at 200F for 20 minutes. Without opening oven bump the temperature up to 225F. for 10 minutes. Bump it up to 250F. for 10 minutes. Bump it up to 275 for 10 minutes. Then BEGIN YOUR ACTUAL CURING TIME. Adjust if you are using another brand to follow the manufacturer's directions for actual curing time and temperature once you start timing the ACTUAL CURING TIME.
The "cookie cutter" type of Christmas ornaments I make to sell are 1/4" thick and that thickness works well for me. BTW, I recommend curing on top of ceramic or terracotta tiles covered with a sheet of copy paper (weighed down with washers if using a convection oven). The tiles moderate the heat. If using paper, you need to move your pieces on the paper just a tad once you have take the curing tray out of the oven, so that the pieces don't stick to the paper as the clay cools and buckle a tiny bit.
I'll try to check back to see if this solves your problem or if you have other questions. Good Luck and Happy Claying.
Anita in AZ