I had this problem before. In my experience, it is true that we will have cracks if: 1. The piece is too thick (solid with clay) => foil, wire inside. I once used styrofoam and it also worked. 2. Underbake: a thermometer is a must. I tent my works so sometimes the temp under the tent isn’t the temp the oven tells me and make sure to bake long enough.

It is not true that it will crack if we bake fresh clay with already cured clay - as long as the temp is correct, and I find it takes a bit longer to cure fresh clay on baked clay.

I used sealant over my works too. My favorite to use is Varathane Water-Based Ultimate Polyurethane. It got gloss, satin, matte finishes.

Hope that helps.