Answer

Greetings.
I went through it quickly, and enjoyed it, as I have from this author’s work for many years. (Though there is room for further editing and correction of typos, and in some places where he was brief he should elaborate more.)
Out of love for sacred matters, I will add a few comments—and once it has come up, I won’t hold back:
1. It would be proper to include a table of contents to make things easier for the reader.
2. In the introduction you connected the Sabbatical year to the destruction of the Temple, but simply speaking it is connected to most of the Jewish people dwelling in the Land, unrelated to the destruction as such (except indirectly, in that the exile was caused by the destruction). In fact, nowadays there are already those who say that most of the Jewish people do dwell in the Land, and very soon it may return to being a Torah-level obligation. Still, it is debatable whether Jewish sovereignty exists today (I tend to think not), since that too is apparently a condition for the Sabbatical year to be Torah-level.
3. Regarding Nachmanides’ view on the prohibition of commerce, it is worth noting that even according to his view, straightforwardly there is no positive commandment to eat Sabbatical-year produce; rather, commerce is a prohibition derived from a positive commandment. In my humble opinion, many have erred on this point. True, we rule that a prohibition derived from a positive commandment has the halakhic status of a positive commandment, but that is only a definition of the halakhic status of the commandment. In such a case there is no positive commandment, only a prohibition. See at length Nachmanides’ glosses to the sixth root, and study it carefully.
4. The Chazon Ish was exceedingly lenient regarding preserving fruit-bearing capacity (that is, where the act preserves not only the tree but also the fruit), and there was room to address this in the discussion of preservation versus improvement. More generally, as is well known, those who were stringent about the sale permit were very lenient regarding labors and prohibitions of the Sabbatical year, because in practice it is impossible to keep Shemitah today according to the strict letter of the law. Therefore the dispute should be presented as a dispute over what compromise is appropriate, because there are no “stringent” and “lenient” camps here. On the contrary, in my opinion those who rely on the sale permit are much less lenient, since once you use the legal fiction, it is a route without real problems, unlike the others. Those who refuse to compromise at all simply do not recognize reality, or do not care about it or about the people harmed by it (how many farmers are there in the old yishuv in Jerusalem or in Bnei Brak?). And those who prefer importing Arab produce so as not to violate “do not show them favor” are downright astonishing. By doing that, they have given the Arabs a stronger hold on the land and weakened the Jews who live on it.
5. Regarding differentiating between categories of labor in the Sabbatical year: the novelty proposed by the Minchat Chinukh requires more detailed discussion (and in general there are parts of the book that are too brief and should have been spelled out more). At first glance his words are very puzzling, since on the Sabbath the differentiation between labors exists because there is a source for it (“one of these” or “kindling was singled out to divide”), whereas regarding the Sabbatical year we find no source. Still, there is room to discuss it, because on the Sabbath the forbidden labors do not appear explicitly at all, only in the general phrase “you shall do no labor,” and the labors are learned from the Tabernacle or from exposition on the word “labor” and the like. So there we need a source for differentiating the labors. But in the Sabbatical year, at least for the labors explicitly mentioned in the verse, perhaps we derive differentiation from that itself, even without a separate source. In truth there is also room to discuss what warning is given before punishment, according to the various views about formal warning for Sabbath labors, but this is not the place. There is also room to discuss the enumeration of the commandments, where the Sabbatical-year labors are not counted separately, and that would seem to be evidence that there is no differentiation between labors (on the Sabbath they are not counted because they do not appear explicitly in the verse, and the differentiation of labors is learned from an independent source).
6. As for hope for the future redemption, there is much to discuss. I am not sure it is proper to hope that the Sabbatical year will return. The world is no longer built on agriculture, and the Sabbatical year no longer has the meaning it once had. There is no point in yearning for reality to go backward (that is not the meaning of redemption). It seems to me more reasonable to update the laws of the Sabbatical year and adapt them to our times, and such proposals have indeed been raised—though from the liberal side, and so they are usually dismissed with contempt.
As is well known, the Sages say that the commandments will be nullified in the future to come, and we can see the process with our own eyes. When meat and milk are produced in a laboratory, the prohibitions of meat and milk will disappear, and the same will be true of the Sabbath and other laws, perhaps including the Sabbatical year. I see nothing wrong with such a process. When our judges return, they will have to decide what is right to do in the new reality, but in our current reality observing the Sabbatical year is simply impossible (it is a decree of destruction for agriculture), and I do not see that changing in the future. So am I supposed to pray that we go back to plowing with oxen and eating whatever vegetables grow in our garden?! That is a very fossilized conception of Torah, and much more could be said, but this is not the place.
 
And I will conclude with the blessing of an ordinary person: May it be God’s will that your wellsprings spread outward, and that you merit to magnify Torah and disseminate it among the many as your good heart desires.