To change a shortcut go file>preferences>key, scroll down till you find the relevant one (edit - copy/ edit - paste) click on it, then click in the white box in the upper right corner above "delete," then press the key you want to bind. There is no eraser tool as far as I know, to erase you want to draw with your transparent colour, I always keep my transparent index in the bottom right corner of the palette so it's easy to find. If you're having trouble figuring out what your transparent colour is click on the "..." icon on your layer and turn transparency off, colour select it (using right click) and then turn transparency back on. I suggest always working in 8bit mode unless you happen to need more than 256 colours in order to make your indexes easily manageable.
Graphics gale has 2 types of transparency, layer and frame. Layer transparency basically lets you composite layers together but won't work for onion skin, you access this in the "..." menu in the layer window. Frame transparency is what you want for onion skin. This is the "..." icon on the frame window. Make sure each of your frames has transparency turned on, then click the onion skin button (the green running man) to activate. Click the arrow on the button to change the mode to either show the last frame, next frame or both frames. Under preferences>view>onion skin tick the "loop the first and the last" box to make the onion skin loop, this is good for cycles. You can also adjust the opacity of the onion skins here.
My personal suggestion is to set a shortcut for "frame - forward frame" and "frame - back frame" so you can scrub (flip through the images) and then use only frame forward or frame back on the onion skin and scrub to the opposite frame when you're in-betweening, having both on makes it pretty much impossible to see.