I have never heard that fair headed people fair worse in radiation treatments overall. It is likely though that fair-skinned people might burn more readily on the exterior of their face or neck from the treatments. As for two weeks being a point where things started to get bad...that sounds about right. During the first two weeks I was going "Hey, this is a cakewalk"... that was until the accumulated radiation started to build up. Radiation sickness, mucositis (mouth sores), thickened saliva, and then the lack of it, difficulty in swallowing, all kick in after the second week. It is going to get worse, but trust me, these painful parts of the radiation process, eventually pass. Everyone is definatly lethargic during the treatments, I wanted to sleep all the time. Remember your body is taking a beating during radiation treatment, it needs to recuperate and rest while it does the needed reapir work. Being tired is normal.
As to getting the feeding tube, read the other posts on this board related to it. Everyone that has had it swears by it. And just as uniformly, they were all reluctant to get it put in. She will reach a point where she is not getting the proper nutrition and will begin to lose weight. When this happens, the doctors will likely put her on it, whether she wants to have it or not. It is for the best. Good nutrition is important to help her body rebuild from the destructive effects of the radiation.