This Just Got A Lot Harder...

Macerated only? Not simmered? ... I would say that had to have been a really young animal. On the young ones, the joints are not fully fused together. So far, an easy fix. Get some non colored rubber bands and put those on it to hold it together until you are done. You can also wrap it good with white string to hold it together. Degrease and whiten and then start gluing that loose parts together with Elmer's glue. Just a small bit is needed and use rubber bands to hold the parts together until dry. Use a toothbrush and straight detergent to do any scrubbing and just be gentle. That back piece you have and the front nose bones coming off are common. Fit the pieces back on while still damp, glue and secure/clamp in place. As the bone dries, the joints will also close and tighten.

 

Can you put the string through the peroxide, or will it react?

 

Cotton should work as well as nylon or other man made fiber. Just make sure it is white and no dye. Cotton will probably, eventually deteriorate but not in the time you will have it in the peroxide. Cotton will stretch some when it gets wet so the man made fiber would probably be better as it will stay tight. I have used uncolored rubber bands or even zip ties with success. The rubber bands I found were white and probably not natural rubber.

 

It looks pretty good. You might want to wrap it with white cotton string all around to hold the joints together tightly until it all dries.

 

It should stay together. If you can get any parts to move, take a toothpick and work Elmer's down into the joints and let it dry. Not sure if doing this after sealing it with Mop and Glow will work as the glue will be on the surface of the sealer and not the bone.
It looks pretty good and they are going to be happy with it. When you go to attach it to the plaque, just remember that there are parts that are glued and don't use a lot of force on it.