Two 7.5-min grids... 31998 x 16002 pixels, so mine is a little bigger. But I'm not covering anywhere near the square miles you are.
It just seems odd that it takes about a second to kick out the blendmask without adding "-i" but I never get past zero percent if I add the invert command. It's not the end of the world that I have to open up Gimp but I am trying to eliminate extra steps and troubleshoot scripting issues that that give me issues like the black and red lines at the borders.
But some unrelated good news is that I am learning how to get creative with ScenProc commands and generate a wide variety of objects like grain elevators, water towers, windmills, storage tanks, churches, and other items based on height and location.