Rivet and Solder too?

After countless attempts to set this stone in a bezel to be soldered onto this pendant (.999 PMC3) I set rivets. Ha! That didn't work either because the prongs flopped around and were unruly. So, how to hold them in place? Solder them to the back of the pendant. This may have been done before, not claiming rights.

Bingo! Sand down the rough edges (Jool Tool) and now ponder if the prongs are too long.