Sorry, I think I need a little bit more clarification. By erase/draw on top of the layer, do you mean the eraser is basically the background color drawing on top of the image, or it actually remove the vertex at that point?
I get what you are trying to do with the 2 triangle thing (or atleast I think I do). What you are trying to do is creating a rectangle (or square) that will draw things in the shape of a square that take more than one pixel. I think I can do that. However I'm still not sure about what i just asked above, as far as I understand, by doing what you said I'm just drawing the color white in the shape of a square over them, which in turn of course erase the bottom layer. But that isn't what I'm looking for however. When I erase something, I want the pixel at that location to be gone, as oppose to being recolored.