Cultivar names, what you are calling a clonal epithet, are used to recognize individual plants. Those names are not registered, per se, unless the plant wins an award. The cultivar name is recorded as part of the award process to recognize that individual plant. If you give a cultivar name and then sell that plant and it is not awarded, the buyer can potentially give it a different cultivar name. If you buy a seedling from someone, then yes, you can give it whatever cultivar name you choose. If you clone that particular plant, then the cultivar name would follow in the clones derived from it.