I'm going to guess that you have an HP scanner. A lot of scanners try to "help" you by adjusting the images. When you tell it black&white, that doesn't include gray so it ignores anything that it doesnt see as black and white. Your color magazine trick isn't that uncommon. I usually stick a color postage stamp on the inside lid of the scanner to do the same thing. I would rather crop and convert than scan everyting twice or more.
If you can resize those images to 610x780 it would be a real help. Once I get it working, I'll make the conversion program available so anyone can do it.