1) "V'zacharti es brisi Ya'akov v'af v'es brisi Yitzchak v'af es brisi Ya'akov ezkor v'ha'aretz ezkor." (26:42) The Shl"H has a famous comment as to why this pasuk is considered part of the tochacha. If a child is raised by bad parents and he/she doesn't have a role model or training as to how to behave, it would be no wonder if the child grows up to be a monster. But if the child has wonderful parents who are excellent role models and the child still becomes a monster, then something is really wrong. The Torah is telling us that had we lacked role models, then maybe our behavior could be excused and the tochacha would not be that harsh. But we are bnei Avraham, Yitzchak, and Ya'akov. We come from the best of the best; we have within us the genes of the best of the best. Therefore, when we misbehave, its an even greater tragedy. WHen the pasuk here talks about Hashem remembering the Avos, it places us in an even worse light and magnifies our sins.
R' Lopian Lev Eliyahu (in the essay 'Ha'kove'a Makom l'Tefilaso' in the Ma'areches HaTeshuvah section in the edition I have) quotes a tremendous question on this Shl"H. In the 10 pesukim of zichronos that we say on Rosh haShana, which do not include zechirot that have negative associations , we mention this pasuk of "V'zacharti es brisi...." Clearly we are quoting the pasuk to elicit rachamim. How can the same pasuk be both a tochacha and a vehicle to bring about Hashem's mercy? R' Lopian in that piece says the answer is a deep yesod that he will get back to some other time -- and then he leaves us with the question. Nu, any answers?
2) "U'fanisi aleichem v'hirbeisi eschem..." (26:9) The parsha until this point contained other brachos as well, brachos about bountiful crops and sufficient rainfall. Why does the Torah only now use this phrase of "u'fanisi aleichem?"
Netziv answers that the Torah here is speaking about raising children, "v'hirbeisi eschem..." When it comes to crops, to rain, etc., it's enough for Hashem to direct his hashgacha to those items at the appropriate time, the appropriate season. When it comes to raising children, it takes constant hashgacha pratis for things to work out well.
I would say that in this regard we have to take a lesson from Hashem. It's enough to check the market once in awhile, but kids need constant hashgacha.
3) "V'im lo tishmi'u li..." (26:14) In many places Rashi writes that "li" means lishma, e.g. Rashi comments on "V'asu li mikdash" that the mikdash must be built lishmi, i.e. lishma. The Netziv similarly here writes that the pasuk is speaking about Klal Yisrael not doing things lishma, for the right intention. This is the start of the tochacha, the first step on a downward spiral of misdeeds. But, asks the Netziv, don't Chazal tell us that, "l'olam ya'asuk adam b'Torah u'mitzvos shel'lo lishma," that a person should do things even she'lo lishma, as that ultimately will bring a person to do things for the right reason, lishma? Don't Chazal tell us that doing things she'lo lishma is the start of an upward spiral, not a downward spiral?
My wife suggested that it all depends on your starting point. If your starting point is she'lo lishma, then doing something is better than nothing and may lead upward. If, however, you were doing things lishma and then regress to a she'lo lishma, that another story.
Netziv answers that there is a difference between the individual and the community. On an individual level, each one of us may do things for less than proper motivations, with the hope of one day doing better. These shortcomings, however, cannot be institutionalized on a communal level. The community needs to strive for the ideal, to strive to act for the right reasons and with the right motivations.