Full cure is when it meets all the final specifications. There might be some out-gassing, but it will be as hard as it is going to be, and adhesion is as good as it gets. Tack-free is as the name suggests, not sticky so you can move it along the assembly process.
The dielectric strength is material intrinsic property and withstand voltage is surface property which depend on thickness of the material. They can be slightly different for thicker materials, but for conformal coating, the two numbers should be very close or the same. That is because we test coating at 3-5 mils thickness, calculate, then report the value per mil.