DR Horton is one of the best and most reputable builders in the San Antonio area.
As with all builder, they specify the materials used in the fences around their new homes, that varies depending on the price bracket of the home. Fence companies comply with those requirements. However, at the time the house was built, it was possible to upgrade to just about anything you want. You would agree that with the builder at the time of purchase or building the home. Fence companies are often asked to upgrade to meet a home buyer's specific requirements.
There are thousands of fences throughout. San Antonio the use the specifications you describe, that last many years. Left untreated any fence, whatever the materials used, will be affected by the San Antonio summer heat and humidity.
Cedar fences are by far the best as they are more rot and weather resistant. They also look nicer. All fences, when treated and well maintained, will give many years of service and look so much better. It is worth the time and investment to do this.
Depending in soil conditions, most fence posts, can be removed by using the top of the post (almost six feet about the ground) as leverage and moving from side to side - if fact that's the best way I know of to remove a post. Concrete should wrap around the entire post. In San Antonio, a typical fence is a solid barrier against the weather. In some weather conditions a fence can be blown down and there is very little anyone can do to prevent it. However, I can tell you, that the type of fence that is typical to this area, for the most part, hold up even on very windy days.
2 x 4 rails are a standard used in thousands upon thousands of fences. 2 x 6 is a very uncommon choice for a rail. For the extra investment it would make a very good fence rail and all fence companies should be able to offer this upon request. But it would usually be a special purchase for them.
You cannot buy 7 foot posts. 18 - 24 inches in the ground is a standard depth and fine for most applications. Posts are trimmed to ensure a standard height for the fence. Trimming posts is a standard practice. If you go 30 inches in the ground you may not be able to have a 6 ft fence, especially if you have a cap and trim. Galvanized steel posts are by far the best choice. As always the case, you get what you are willing to pay for or what you are able to afford.
All posts and rails are treated. Anyone who has a fence or deck understands that with wood being a natural product some pieces will warp, even when treated (my own deck is a good example of this). If this happens in any extreme, call the fence company and they will replace.
Five nails are always used per picket; two at the top rail, two and the bottom rail and one in the middle rail - standard practice. Any standard practice can be upgraded upon request.
You live in a housing development that by now is more than three years old. Most fence companies will service their fences even past the one year warranty, I would suggest a call to the fence company and see if they could assist; even after warranty has expired they may still fix for you, or at least recommend the best course of action.
If you are not happy with the fence your builders recommends, ask for an upgrade at the time your home is being built, including a fully treated fence which is easy to do, that might save the cost of replacing a fence and get you the design and specification you prefer up front.