Your initial claim is "they could become insanely rich in a single day", so point 2-4 matters a lot to them (the one who perform double spend). And about point 4, exchange definitely have some data of account used to short BTC.
The doublespend is only to destroy trust and cause a collapse in anything connected to btc,
($100 or $1000) effect on trust will be the samethe money can be made in shorting options listed above , not from the double spend itself.