A lot of controversy regarding the loopholes and solutions for Shemitta has occurred this year - my question is, why isn’t the Law of release of debts on the 7th year applied today, since it is obviously part of the shemitta year. And if it does apply, is the prozbul still valid? And if the prozbul was acceptable, why is the hetermechira scorned upon?

The laws of release of debts (which takes place at the end of the shmitta year) do apply today, and the pruzbul is still valid. Your question about the difference between pruzbul and "heter mechira may be approached from several angles. Pruzbul is mentioned in the Talmud and required the enactment of a law (by Hillel and the sanhedrin) to apply, whereas the idea of selling land in Israel to non Jews to avoid shmitta is not mentioned or suggested in the Talmud or in early sources. Furthermore, selling the land in Israel generally is prohibited, while pruzbul requires no prohibited act. (Those who accept the "heter mechira" can show numerous sources that a temporary sale of land in Israel for benifit to settlement of the land is permitted, but it is at least a bone of contention). However, you have a valid point that there are paralels between these two ideas.