I appreciate you posting the vet's letter Stormi and certainly don't want to add to your grief. However, for informational purposes for others who may read this thread at a later date, the information presented in the vet's letter does not provide any answers as to the cause of Teukie's illness and death.
While I am not a veterinarian, my first thoughts on hearing his symptoms, later confirmed by the vet's letter, is that there was a disease, parasitic, or exposure to toxin process going on. Biopsies would have gone a long way toward solving the mystery. I understand that some people won't care what the cause of death is, that dealing with the loss of a loved family member is all that matters. But in homes with multiple cats, I would encourage anyone facing a similar situation to Teukie's to go further to find the answers. It could be important to the health of your other animals as well as any new animals brought into the home.
There is something not adding up about Teukie's situation and I would encourage all of you who may face such an awful situation in future to challenge and question your vet regarding diagnosis and treatment.