Random items that I noticed after attending a recent weekend workshop and conference examining 19th century fabrics and dresses: The gauging (cartridge pleating) on your dress is excellent. The piping at the waist, however, seems quite a bit thicker than on most Civil War era dresses--at that time a very thin string was used. I'm wondering if the original waist piping was replaced later on? Is the piping on the armscye and collar the same size? Or is this just a camera angle issue?
I see no evidence that this was a two-piece dress. In normal dressmaking technique for the period, the skirt and bodice were each completed and then whip-stitched together. Except for a very few cases in which a ball gown bodice and a dressy day dress bodice shared the same skirt, nearly all dresses of the era were one piece. Very late in the Civil War, the separate blouse and skirt became an item for fashionable young ladies, but they weren't very common. (For any young woman past her early 20's, the separate blouse and skirt for the Civil War era are a modern reenactorism.)
The collar may (or may not) be a later addition. Normally, collars were basted in and changed frequently, usually daily, to protect the neckline of the dress from getting dirty. (The same was true for cuffs and undersleeves.) Nearly all collars had bias binding attached which was basted to the inside of the neckline. I note that some hooks and eyes have been replaced or at least resewn with different color thread, not unusual. Notice the hem braid to keep the bottom edge of the skirt from fraying.
So many mid-19th century dyes faded to brown (pink and purple were the worst), that you really need to look at an interior seam allowance that has not been exposed to light and air to find the original fabric color. One thing I found out at our conference is that my camera did a lousy job reproducing original fabric color, especially under fluorescent lights!
I didn't take the workshop on preserving original garments at our Oregon City Conference because I don't own any originals and probably never will. The only advice I can offer, therefore, is to keep the garment out of direct light, covered up with a cotton sheet when you're not admiring it, while you research thoroughly how to preserve 150-year-old fabric while this dress is still in a good state of preservation! Always handle it with clean cotton gloves, and avoid dragging the fabric against anything, including itself, when you move it.
If there's a museum anywhere in your area that has 19th century clothing, you might consider making an appointment to have the dress looked at by an expert, which I am definitely not!
I'd love to see more details of the back and armscye, should you decide to take more photos!