What exactly are enantiotropic and monotropic polymorphic transitions?
If you can get your hands on a copy of Fegley's book Practical Chemical Thermodynamics for Geoscientists, he gives a good description in Chapter 07, which I'll summarize here.

An enantiotrope is a polymorph that undergoes a reversible transformation into another polymorph at atmospheric pressure. If you're not familiar with the concept of reversibility you may want to read up on entropy, spontaneity, and the 2nd law of thermodynamics. Briefly, a reversible transformation is a process in which the system and surroundings are in equilibrium at all times. An important property of these processes is that $\Delta$ S$_{universe}$=0. Reversible processes don't actually occur in nature but they can make good approximations for some things we observe.

A monotrope is a polymorph that does not have a reversible transformation into another polymorph at atmospheric pressure. One polymorph is always stable and the other is metastable. Monotropy exists in systems where the transition temperature between solid polymorphs is above the melting point of the lower-temperature polymorph. In many cases, upon heating the metastable monotrope melts, then resolidifies to the stable monotrope, which melts at a higher temperature.

Metastability refers to a nonequilibrium state that is approaching equilibrium slowly enough that we can say it's stable over a given period of time.