Step 1: Pewter experiments
Video of melting pewter using a steel ladle as a crucible (it worked well for me due to the long handle):
Video of scraping slag off the top of the molten pewter:
Video of pouring pewter into a flat mold:
An additional problem was the pitted, moon-like surface result I got using untreated silicone. I had theorized that the mold was too cool and tried boiling it prior to pouring, but that did not help.
Taking advice from windswept and interesting's Simple Pewter Casting instructable, I tried using my 1-piece mold propped upright and backed with hardboard, to allow gravity to push the molten pewter fully into the mold. From gg1220's excellent signet ring instructable, I learned that a smooth pewter surface was possible by using a graphite powder mold release. I don't know why this works, nor why the surface is so pitted and lumpy without it; if you know more about this, please comment! Graphite powder can be purchased, but I just ran a pencil over some sandpaper for a couple of minutes, catching the result on a small plate. Prior to each pour, I brushed some graphite powder on the inside surface of the mold using a small paintbrush.
These changes allowed me to make some molds and castings I was satisfied with. The next steps will show the details of the successful process I worked out.