I know how heart breaking it is to lose your cat that you love dearly, my 10 year cat passed away in Jan and our whole family including our other cat grieved for long time.
I can’t say anything about the incident of your cat in treatment, I can only rest your fears on the picture of the cat you posted above, I adopted her the same day she was put up for rehoming.
From your description, your cat is male, the cat in the picture whom I adopted is female.
According to the vet, the person who put her up for rehoming paid all the bills for her boarding, checkup, vaccinations and spay. I did not pay them anything except for the microchip which I requested.
I can send you more pictures of her if you want to see the difference.
Hope that you will get a good explanation from the vet regarding what happened to your cat.
Take care.