FWIW, in most US states it is legal for a lay person to give another person a shot of medication. I give my DW B12 shots every 2 weeks, our Dr and Express-scripts is fine with our arrangement. Injection syringes are sold over the counter at drug stores, but you do have to sign their accountability book and show your driers license, which they record in the book.
Same should apply to pets IMO.
As to a deceased pet, I'd do the same as for me when I die, direct cremation.