When viewing an ambrotype, rotating the image will not allow you to see a negative image. Don't mistake a reflection in the cover glass as a reflection from the emulsion behind the glass.
The metal sheet on the back *could* be part of the original preserver in which the image was packaged. The front part of the preserver been been removed -- you can see the outlines. The ornateness of the shape indicates the 1860's. Earlier preservers were much plainer.
Daguerreotypes were the main form of photography until the mid 1850's when the Ambrotype was introduced. It was less expensive, the finished product was not as heavy, and it was popular until the mid 1860's.
I feel your couple's clothing and the woman's hairstyle looks to be around 1865, which would date it after the Daguerreotype period. Do you know where the photo was taken, i.e urban or rural? That would affect both the process likely to be in use and the dating of the clothing.