can i use swan dive to stun ultron and use his ability discard a card, lose 8 and still stun a 7 drop

Costs cannot be replaced, so Ultron's actually going to stun if you use him to pay for Swan Dive.

Well, technically you COULD use Swan Dive on Ultron, then use his ability in response (i.e. discard a card, lose 8 endurance, and Ultron doesn't stun).

Of course, if you do that, then you fail to meet the cost of playing Swan Dive, and the plot twist does absolutely nothing. In essence, you end up losing two cards and taking 8 endurance loss for absolutely nothing.

No, you cannot do that. You either stun Ultron to pay Swan Dive's additional cost, or you don't; replacing that stun isn't an option of any sort.