Marty,
It seems as if your son really knows his early Scottish history. The Davidian Revolution is one period I am interested in as well. I'm sure he is familiar but if not POMS ( people of medieval Scotland) is a great online tool. The information you presented on Waldino de Dundemor is quite interesting to me. The only record I have found of this man is from 1199-1202. I have never heard his connection to the Bissets or Warenne families. I believe the Muriel you speak of is a women. Do you know who any potential fathers would've been? Any resources you are willing to share? The closest date of the seal is mid to late 13th century. From what I can tell it most likely relates to pilgrimage to Spain. Possibly for crusade against the Moors. Just speculation. Another note that I find interesting is that if I am correct and the Dinsmore/Dunsmore is connected to Dundemor then the Ydna associated with them is predominantly from France and Spain long ago. Many anglo norman names share a similar haplogroup as well. If you would like a picture of the seal feel free to attach an email address.
Richie