If Astrum made peace with Caerwyn and they got to keep Golden Farrow that would just be bad. Caerwyn would eventually retake Itau. I and my nobles would leave Astrum. They would be down to 40 nobles or less. Then one day Astrum and Caerwyn will be at war again. It is inevitable. Caerwyn will be much stronger than it was this time since they have 3 cities now and not 2. I doubt Libero Empire or Summerdale will help in that next war and Astrum would have an annoyed Allison pulling strings in the background.
So they could do it and suffer little to no consequences in the short term, but in the end I think that would be a big mistake for Astrum to make peace and let Caerwyn keep Golden Farrow.