I would think with the metal roof the snow would slide with a little push from behind with shovel, little at a time of course, turn the machine around and catch the falling snow so you don't have to pick it up from the ground.
I like the idea of force heating from the bottom.
As far as the melting cables go, I'm on alot of roofs and when ever I see them they are usually a mess and not working, about 95% of the time.
So I would normaly say forget about them, but this year has forced me to rethink things and am seriously gonna look at installing on my own house. I've had repeated ice dams even after removing fresh snow.JB.