After a weekend of play, I decided using the cow plant on a residential lot was too time consuming for my intended purposes.
So I used SimPE to move up the ages of my Sims who had transitioned before I installed the elder hack, then had my cow plant eat Dr. Death - but not before I had moved in a Uni graduate from my Sims bin, who needed a residence and some $$$.
Incidentally, Dr. Death's headstone never showed up at Peaceful Dreams Cemetery (downtown) so am guessing OMGPetz screwed up that option.
The (unmilked) cow plant is still there, so I'll try to track how many days pass before it offers the cake again.