I understand what you’re saying, but the one defining factor is her requesting I make them food.

For the occurrence you referred to in your first post today, did she ask you to make this extra food for them today or did you offer to make them food and she said "yes"?

You can communicate to her how strongly you feel about wasting food, and that if she explicitly asks you to create food that you then have to throw away because she doesn't do her part in taking it to coworkers, you are being forced to do something you hate because of her carelessness and how frustrated that makes you. You can even put boundaries on what you're willing to do:

"I love you, and want your coworkers to be fed, but it pains me deeply to spend the effort of cooking then be forced to throw that away. It makes me feel like you don't care about my efforts and it feels like disrespect from you to me. In the future I will wait for you to explicitly ask me to make a meal for you co-workers, and if I do that and I have to throw it away again, I may not be willing to cook for your coworkers anymore."

and I’ve always been under the idea that if you microwave food and don’t eat it all, the rest is garbage.

I always aim to avoid that altogether making enough for multiple people to eat after microwaving it, but when she doesn’t take the prescribed leftovers, it’s going to be heated up once, and then it’s garbage after that

I'm not sure where you picked that up, but I've never seen a limit on the amount of times you reheat. The only limits are consuming leftovers within a few days. Now I'll do some solutioning: If your concern is on the number of reheats, what prevents you from dishing out a smaller portion from the refrigerated leftovers and just eating that portion? The rest of the leftovers haven't been subjected to any reheating events yet.