Something does not add up.
If the battery is under the seat and the metal seat made contact with the positive post of the battery when you sat in it, then that is when the short circuit would have taken place instantly. There was no need to turn the ignition key in order for that short circuit to take place. The fact that you blew the main fuse also tells me that you have not found the true problem here. Fuses only blow when the current passing through them exceeds the rating of the fuse. In order for that to happen, the short would have to occur somewhere beyond the side of the fuse that feeds the ignition switch.
Think about it.
Either something else is going on here or you have not told this story as it actually happened.