I am so sorry to hear of all you've been through. It is very rare for radiation to affect the blood vessel as you describe but it can happen. I have only seen it once in 16 years. That being said, sometimes there are ways to handle the blood vessels differently that can produce a solution where difficulties have occurred in the past. It is unfortunate that the DIEP was harvested before the condition of the vessels in the chest was recognized since the tummy donor fat is such a valuable source of tissue. I have seen just about every situation there is, I think, after things go sideways at this stage in my career and there may be a way to help you.
Anyway, it may be that vessels higher in the "chain" can be sourced along the sternum or that the vessels in the opposite side can be used as an option by tunneling them across the midline beneath the skin. Sometimes the thoraco-acromial set can be ample. Hard to know, does appear that he has exhausted the "thoracodorsal" set as well which could have given us the latissimus as a backup plan. Will require a bit of thinking and drawing deep from the well to give a real idea of what can be done and that can only be determined at this stage with a personal visit with whomever you decide to seek counsel from.
I hope some of this makes sense and is a foundation to get you taken care of.