Rrrrrrr, need help on this one.

Here is one thing you could try. You claim that if you operate the starter then shut off the key, the fuse blows. Try this. Obtain a large wattage bulb. Like an old headlight or a stop/ tail lamp (1157, etc) Disconnect battery ground and hook the lamp in series. You will not be able to operate the starter. But mimic it. Twist the key to "start". This should cause the test bulb to light brightly. Let off the key to "run." Light should drop down to "not so bright." Now shut off key. Lamp should go clear out. If it does not, and or gets bright, you have a short. Now you can look around and find it. Try disconnecting the alternator, removing fuses, etc