Success! Ok, after the 2nd attempt, but still, I'm much happier that I got a workable positive out of my problematic first mold. A few mental notes: use larger pour holes and keep in mind the 3 minute pot life next time. I can now machine this version and further refine without worrying about all that chipping and instability from my fragile Sculpey original. Also, the next mold will be created with firm acrylic walls and be better leveled for a tighter seal.
EDIT - I forgot to mention that after consultation, I built up the front side of the mold with additional silicon material. This allowed me to move forward and get a workable positive!