I have been lurking in these threads for weeks now. I finally found something to comment on!
As far as finding poisons in an embalmed body - there is a lot that science can do now to find traces of poisons in tissues and hair. Just because a body has been embalmed does not mean that it won't have traces left behind or evidence of the presence of poisons, such as damage to the lining of the stomach. Hair will hold onto a lot of chemicals for long periods of time as well, although it can become contaminated during the process of moving and burying the body. Any tissues left behind can have traces of chemicals that can be found through various chemical techniques like gas spectrometry.
Arsenic in particular causes bodies to decompose differently and at different rates than bodies that were not poisoned. There have been experiments run that show adipocere forming in the soft tissues of bodies that were previously given arsenic. Adipocere can actually preserve tissues within the body like the person had just died, so the tissues will still look red and fresh. Arsenic also has been shown to allow bodies to dry out like what happens with mummification.
If needed, I can get out my medicolegal death investigation textbook to look up information for us when needed. I just finished getting my MS in Criminal Justice with a specialization in criminal investigation. I focused a lot on death investigation while in school, so this topic interests me greatly!