Now onto my question finally, do you think NMM is required for GD, or do TMM with good shading and blended highlights also have a place?

NMM is most certainly not necessary to include in your entry. No specific style is necessary because at the end of the day, it's a painting competition, so it's about what fits your piece and how well it is executed. Looking at the winners on the GD website is always a good way of reminding yourself of the variety of styles that have placed.

Best of luck with your entries!