The melting points of various testosterone powders are different. Some types of testosterone powders will melt on a very hot day eg 40 deg C. They seem to stay as a hard toffee like substance even when they return to a more average temperature. Can they be returned to a powder again, I don't know. Perhaps if they are chilled, or frozen first, then they might be able to be reground into powder. I haven't tried it.
Yes, all melting does is make the powder liquid or pliable. It makes it quicker to dissolve into a suitible solution( with an oil, and solvent like benzyl benzoate) ready for injection. It is not necessary to melt either to dissolve it, or to ensure sterilization. A correct mixture of powder oil, and solvent will dissolve it, especially after it has gone through a filter(which will sterilze the solution).
Melting points are used to verify if you have been actually given the correct steroid powder, or have been given a cheaper substitute steroid, or perhapseven an inactive ingrediant.