> I'm still having trouble with the concept of the blessings being blessings only to a "nation" and somehow not individually directed and also not applicable to Christians
The societies of the OT were definitely community-oriented. Many elements of ancient life were seen in light of the whole rather than as individuals, as opposed to what things are like here in America where we are radically individualistic. You'll notice in Joshua 7 than when Achan sinned (at the end of Josh. 6) it affected the whole country. Many of the commands, injunctions, or judgments of the OT are directed at Israel or at other nations. A quick scan through Isaiah will reveal the truth of that.
> The plain language seems to suggest this is a blessing to "people" (i.e. in addition to any blessings to the "nation")
Not so. Dt. 27.9 shows that Israel is "the people." So it's not as if he is speaking to individuals within the nation, but to the nation as a people group. While it's obvious that a nation is made up of individuals, and the individuals are who must obey, He is addressing and mandating them as a nation, a "people of God".
So also with the curses. The curses here are not statements of what will happen to the one breaking the covenant, but statements calling down unspecified curses on particular types of covenant-breaking conduct. This section constitutes a solemn oath entered into by the people (the nation as a whole, as evidenced by the repetitive corporate "amen" at the end of each) concerning secret violations. Such oath-taking ceremonies regularly accompanied international treaties. They were pledging themselves as a nation, as God's people, to follow the covenant.
> I am having trouble understanding why that cannot be as true today as it was when the blessings were originally enunciated.
Blessings are true today. The New Testament is full of them. Jesus' beatitudes at the beginning of Mt. 5 are a perfect example, but there are many others. God does intend to bless us, and he does bless his people.
> Sometimes it almost seems like Christians get into a persecution complex and don't want to accept that along with their spiritual uplifting, their physical day to day lives can be enhanced and bettered through faith as well.
We have to have a balanced and Scriptural perspective. While God promises blessings (Eph. 1.3 et al), he also promises a life of suffering (Phil. 3.10 et al). Sometimes we experience God's blessings, sometimes his discipline (Heb. 12.5-11), and sometimes great suffering (Heb. 11.35b-38 et al). There are no guarantees from God that life will be sunny and filled with health and wealth. Some do get to experience that, but others are persecuted, tortured, and martyred (
http://www.persecution.com, The Voice of the Martyrs).
As far as Commandment #5, the promise of long life is a proverb, not a guarantee. Long life was believed to be the result of knowing God and following his ways, and being righteous. People who honored their parents were not being guaranteed a long life, but clearly such was seen to be a clear sign of God’s blessing. Anything that brought long life, in their minds, was a sign of God’s blessing for a life well-lived. While long life is not automatically a sign of God’s blessing, one could hope that as one lives God’s way, the result will be long life and prosperity. All things being equal, those who honor those God has put in authority over one in life will live longer than those who spur authority and rebel against it.
As to Galatians 3.29, the promise to Abraham that is the point of Galatians, and Paul's point, is that of salvation by faith. It is justification through Christ by faith, not by obedience to the Law. As Hebrews 3-4 makes clear, the "rest" we enter is not that of land, barns, crops, etc., but salvation and the heavenly rest in which God dwells. We share not in an earthly kingdom, but a spiritual one (Heb. 3.14). The promise of entering God's rest still stands (Heb. 4.1ff.), but it's the kingdom of God, not the physical and material blessings of the old covenant.
What kind of heirs are we? We are heirs of a relationship with God (Gal. 4.1-7). We are justified on the basis of a relationship, and not anything else.