Chazal say that the first Parsha is to warn the yachid(individual) and the second Parsha is repeated to warn the tzibbur(the community). (Rashi, on second Parsha, Deut/11/13). The question is asked, (I think it is brought earlier, but I just found it quoted in Artscroll from Maskil L'David), "Why does it say you must dedicate 3 things ["all your heart, all your life and all your resources"] in the first and only 2 in the 2nd[leaving out "resources"]? The answer given (I think it was said by earlier sources but Artscroll attributes the answer to Maskil L'David), "Only an individual needs to be warned to [also] give up resources, since there are yechidim who would be foolish enough to think twice if someone said 'your money or your life.' A tzibbur would not and therefore needs not be warned, once it has been warned to give up its lives."
Now, we need to see why each time the second Parsha switches away from plural, which is the given, to singular. I have put a lot of thought into this and only found One or two sources that supports part of my meanderings. First we must posit: is there a connection between the 3 things and tefillin and mezuzah, which follow, in the first Parsha? It seems that each of these Mitzvos are a way to help us out with the 3 things mentioned. (1) we wear a tefillah on our arm, near the Heart, to strengthen our resolve to sacrifice our heart for haShem. (2) We wear another tefillah on our head, near our brain, which controls our Life, to strengthen our resolve to sacrifice our life. (3) what, then, would be the best way to strengthen our resolve to sacrifice our resources? By putting on a mezuzah to dedicate our most dear, expensive resource, ie our Home! I searched all over and finally found this in "Emes L'Yaakov" by Rav Yaakov Kamenetsky, who quotes "Talmidei d'Rabbeinu Yonah" that say this!
According to this comment, we would like to refine our question: "If the Maskil L'David is correct, why is there a need at all, according to the Talmidei d'Rabbeinu Yonah, to mention mezuzos in the second paragraph? Mezuzos are given to strengthen a weakness of valuing money over life! And that, according to TDR"Y, is not necessary in the public sector! Can these two commentaries be merged into one?"
Now, with that intro, let's look at the second Parsha.
The Second Parsha is known as the Parsha of Kibul Sachar (receiving reward) for following the Mitzvos. It starts off by announcing that, if the tzibbur follows "with [all of] your hearts and [all of] your lives, then I will give the precipitation of your [community's] land in its appropriate time, early 'shooting' [Strong] rain as well as late [gentle] rain."
The narrative now switches to the singular to stress and ensure us that this will not be a communal bracha, possibly leaving some to beg from others better blessed, rather it will be a bracha that is bestowed upon each and every individual!
"...and [each of ] you will collect your [individual] grain, your [individual] old wine, your [individual] olive oil." And "I shall give herbage in your [individual] field for your farm animals; [afterwards], you shall [each, individually,] eat and be satisfied!"
So, although the rains come down to the whole land, the bracha is sent individually!
Now the narrative returns to reinforce that we are talking about the whole tzibbur:
"Be forewarned for yourselves, lest he sway your hearts and all of you will turn away and you will all serve other gods/forces, and you will all prostrate yourselves to them.
Then the anger of haShem will be heated against all of you (Chas v'shalom!) and He will withhold the Heavens and there will be no precipitation and the Land will not give forth its produce. And you will all be lost quickly from being on the Land which haShem is giving to all of you.
Therefore, [to strengthen your resolve], all of you shall put these, my words, upon [each of] your hearts and upon your life forces, [BY] tying, [all of] you, them (these words) for a sign [to strengthen your resolve to give over your hearts] upon your [arms, near your hearts] and they (these words) shall (also) be for reminders [on your brain, which is your life force].
(Btw, I found that the Baal haTanya says similarly in the Shulchan Aruch haRav)
Now we have another switch, mid-posuk, from plural to singular.
"And [all of] you shall teach them (these directives) [to] [all of] your children to speak with them [ and to show your commitment, you may not depend on the tzibbur to transmit these directives to your children, rather EACH OF YOU INDIVIDUALLY shall teach them, with [each of] your sitting in your [individual] home, and while you are [each on] your [individual] path, and when you [each] lie down and when you [each] get up."
Now since, when discussing teaching the tradition to our personal children, we have stressed the individual's PERSONAL responsibility to teach, we have returned to a situation when we can no longer depend on the tzibbur. We've returned to a situation when an individual might transmit the [wrong] message to his children that we should give up our hearts and lives but perhaps NOT our resources. Therefore, we now state in the singular, "And you shall [each] write them (these words) on the mezuzos of your [individual] house and [individual] gateway."
(l'ma'an) If you do so, teaching each of your children to give over heart, life, and resources, to haShem, then (yirbu y'meichem...), ALL OF your days and the days of ALL OF your sons will be increased on the Land which haShem, the Powerful G-d of ALL OF you, has sworn to give to you like the days of the Heavens on the Earth!"