Those windows and frames look great! Yes, the mullions are so fine it would be easy to break while cutting the frets. I like your drawing in the first picture. Though I helped my dad rebuild double hung windows in his 1888 house we both grew up in, your drawing helps visualize what you are doing on the model.
Has your grandson picked a college and major yet? It's a question I always ask. I find it exciting to watch these young people become adults!!
