I understand that it will be stressful to see such a step back for Loki. Now based on what you have said, it does sound like she may have had a severe gastrointestinal upset with this spoiled food. And if her nausea was severe and her vomiting quite violent, then that may be why she cried last night. Though at the moment, it sounds like she is settling already if she is comfortable and purring. And if this is just due to that food being off and triggering severe GI upset, we'd expect her to start getting back to normal over the next 24 hours.
At this point, its good that she is on the Remeron (Mertazipine). Though if you find that she is struggling to eat even with that or vomits any more, then I would suggest speaking to her vet about combination treating with a stronger anti-nausea medication (ie Zofran, Metoclopramide) or even considering an antacid (ie Pepcid, Zantac, etc). If she is responding to the Remeon just now, we don't need to rush to use those as well; but it is worth having a back up plan if we see any more vomiting.
Overall, we can certainly see spoiled food cause the signs you have seen. Though if it is our trigger, it does only tend to be for the short term. So, you are doing perfectly here with the light diet and Remeron. So, we want to keep up with those and offer her food in small frequent meals. As long as there is no vomiting from here on out, we'd expect her to get back to normal over the next day. But any more vomiting or if her appetite doesn't pick up, then we'd want to consider speaking to her vet about those other nausea treatment options to ensure we nip this in the bud for her as quickly as possible.
All the best for Loki,