What are the Orange things, registration keys? I've never seen them done that way.
I would try sliding a strong, flat tool (like a wax tool or HEAVY clay knife) between the layers of rubber on the edge and try to pop it open enoigh to break the seal. Work it around the edge until the two halves are separated. Then I'd use a blunt tool strong enough to be hammered on and put that against the rubber in those nipples. Tap with a mallet or hammer to push it out of the hole. You may need to cut off the end and/or run a flat tool
around the rubber to break the seal. It may take some time, but you should be able to free your piece that way.
The way I've always made molds and seen them made (by my bronze and reesin foundries) is to put a ridge of clay around yoursculpture at the parting line (where the parts of the mold come together). You can but plastic buttons, acorn nuts, etc., in this clay to be your reistration keys.
This Smooth-on video shows you exactly how to make a mold. The acorn nuts are the registration keys. I make my mold boxes out of foam-core, duct tape and clay. I seal all theseamsinside and outwithduct tape, then smooth plastilene over the tape sealing the corners and bottom edges. I use an Exacto knife to cut them open, then reseal as above and do what is shown in the video to pour the second side.
To make the mother mold,do what you did but without nipples. If the mold is large enough to need them, drill holes through the edges (when dry). Or you can wrap it with a heavy duty rubber band or a cut bicycle inner tube. Here's a tutorial using the mold material you used:
Hope this helps!