The question almost contradicts itself.
I'm DMing a Tomb of Annihilation-based campaign and we're using Meat
Grinder Mode (DC for Death saves is 15).
ToA is designed as a more lethal module in the first place, and the meat grinder option is deliberately built to be more lethal than normal 5e.
You then ask how to make it less lethal, based on your problem with death saves, and your desire to attack characters who are at 0 HP since that does indeed fit the situation that your characters were in. (Quite frankly, your ruling on that makes perfect sense to me, in terms of the beasts involved and the situation the characters were in during that fight).
It's kind of like "how do I go whitewater rafting but instead go canoeing" as a question.
Now that you have edited the question, and I have offered an answer, I still perceive a modest "XY problem" in the question. Since the question is open again this meta appears to have been overcome by events.