Well from my experience what you should do to avoid sinking is pour a small layer of melt and pour first. Wait for the to be solid enough to place the shapes and then resume pouring! It creates a table that "suspends" the soap!
Also the issue with melting embeds seems to be a temperature issue. Make sure you aren't pouring about 130 degrees F. at maximum. The 120s is better especially if the pieces and shapes are smaller. I personally don't recommend freezing the embeds because that can cause them to "sweat" which can look kind of icky...
You should try watching the SoapQueenTV YouTube channel! It has tons of tutorials and techniques that could really help you out!
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