Another area I have researched quite extensively is elastic. I found that it did, indeed, exist during the Civil War. So I occasionally use it in certain patterns to give a secure fit across the shoulders.
When it comes to boning, plastic stays are the most commonly available on the market. Being sewn inside the gown, they are never seen and the plastic makes them unlikely to break or, worse, puncture a corset-less torso.
As for seam construction, I serge most of my internal seams, because it makes for a much nicer gown. I like a gown to be as lovely on the inside as it is on the outside. I also usually use a polyester/cotton thread, because it is less likely to break.
Where buttons are concerned, I often use metal and occasionally I use wood. And, by the era of the Civil War, lacquered buttons had become quite popular. They were made from a metal base which was repeatedly dipped in a lacquer to which dye had been added. Such buttons are impossible to find today, but many modern buttons look virtually identical from the front to these lacquered buttons of old.
Civil War Reenactors often have different and very specific needs. They usually require gowns that are period correct in every respect down to the smallest detail - a dress like one that Elizabeth Keckley, the seamstress who dressed Mary Todd Lincoln, would have produced around 1864. Such a dress would have been sewn with period correct fabrics, such as cotton, silk, wool, and linen. It would have had grommets, metal hooks and eyes, or hand-sewn buttonholes for back closures, and all hems would have been done by hand. The buttons would have been metal, wood, or bone.
In sum, I always do my best to deliver a gown that is stunning and memorable and also affordable. If I am creating a custom gown, I work with the customer to achieve whatever level of period-correct accuracy they need.