So upon thinking about what might have gone wrong with the soap cake and what to change for next time I make a soap cake...
1. The soap cake patties popped out of the silicone mold and were smooth as glass. Actually really pretty, but probably too slick for this purpose. The surface was too slick for the frosting to stick well.
1a. Next time, rough up the surface: scratch it up with a fork or something. Like in pottery, scoring the edges before sticking them together.
2. Gelling, I didn't make any particular effort to gel the soap cake.
2a. Warm the oven up to do CPOP. Gelling will allow the old soap patties to heat up and hopefully meld better to the new soap.
3. The soap cake is really HUGE, well at least to me. 5lb of oils/butters or 7lbs total weight. I had a hard time deciding on color and fragrance for such a large amount of soap.
3a. Maybe next time I'll just use a log mold that holds one or two pounds, cut the soap log down the center laterally to make two thinner rectangular patties and make, not a round cake, but a rectangular cake. I could still frost it, but it would be a smaller cake and the slices would be rectangular/square rather than wedges which seems more of a challenge to use in the shower.