Many of the passages in the Parsha of Nitzavim deal with the nature of Torah, the philosophy behind Torah, and the outlook we should have when following Torah. Some of these passages seem almost contradictory - the man who blesses himself saying that "Peace will be with me, though I walk as my heart sees fit" is cursed. What does the wording of the phrase "Adding the watered upon the thirsty" mean?
Rabbeinu Bahya says that this refers to the insatiable urge to do sins. Once he does one sin, he becomes thirsty for more. I think it could be referring to the idea that, he may be doing some mitzvot, but he only does them because they feel convenient for him, not for the true reasons. Therefore, he does have some element of being watered by the exposure he has had to learning Torah. He has been exposed to Torah, and instead of being a beacon of following the commandments to his best of his abilities, he simply follows it only when it is convenient to him, only when he personally thirsts to have the feeling of having done something good, rather than because he really wants to serve Hashem.
There is a difference in Rashi versus Rambam in the interpretation of this verse. Rashi holds that this means that sins such as idol worship that are committed in secret are not the responsibility of the community to publish, but sins that are committed publicly are the responsibility of the community to punish. Rambam has a different idea, that the secret sins are sins that even the person doing them is not aware of, and the revealed sins are sins the person doing them is conscious of.
I think one other interpretation is that there are secrets of the Torah that belong to Hashem, but by revealing and bringing light to aspects of Torah that we may struggle to understand, we make our portion in Torah for us and our children.
Many rabbis interpret the idea of circumcising of the heart to refer to removing the desire for forbidden things, especially sexual forbidden acts. That, like this part of the body is removed in circumcision, Hashem will purify us of the desire to engage in acts that He has forbidden.
I think this could also refer to removing the thoughts that stop us from doing teshuva, removing our internal justifications for ourselves for why we feel we cannot do teshuva - we feel we are too far gone down a path, we worry our teshuva will not be accepted, we feel we are not doing anything really wrong. These justifications, these barriers to us going through the process of teshuva, will be removed so we can get closer to Hashem our G-d.
This is the famous passage that is quoted in the Talmudic discussion of the Oven of Akhnai. Here, it is used to shut down the use of heavenly miracles in a Talmudic discussion, relying instead on the Torah principle of following the rule of the majority.
I think it can also mean that you cannot shut a person out of their relationship to Torah. Even the most simple seeming person has a deep relationship with Torah and deep Torah insight. Someone who might seem incredibly holy is not the only person who can shed light on Torah. It is not just the person who has gone to the greatest lengths to learn Torah who Torah is for. It is for all of us, and all of us have a contribution to play in understanding it. It is best understood in a community, where all people's views are listened to.
I think what it means to call heaven and earth to witness that these options were put before us, is that we cannot say that the Torah was given to us as a instrument of harm, that harm we do and justify based on the Torah was something Hashem wanted of us. Hashem commands us to choose life, to use the Torah as an instrument of life, and to protect our lives and the lives of others. If we do something, based on the Torah, that makes it harder for ourselves or others to stay alive, this was a choice granted to us, but it wasn't how Hashem intended His Torah to be used.