Your minimum instances would always be 1 in the autoscaling group associated with your ELBs (or now, ALBs).
Regardless, neither the Internet Archive nor Wikimedia use AWS or other cloud providers, as it would be prohibitively expensive. They both run their own infrastructure/ops.
I think a reverse DoS attack would be one that actually increases the capacity of the service. I'm not sure what scenario would allow that to happen though.
Some streaming sites (e.g. 4 on Demand, IIRC) have a peer-to-peer element; you could reverse-DDOS those by running a lot of computers "seeding" (perhaps by leaving them on the page but at the end of the video) distributed all over the globe.
What would probably happen between 1 and 2 is that archive.org notices spike in traffic from Wikipedia, talks to Wikipedia engineers, they find out who is generating all these requests, and block them from editing Wikipedia.
Though it's certainly possible to generate enough junk edit traffic to cause disruption on Wikipedia, but that's nothing new. It's the nature of Wikipedia as the resource - it trusts the internet community to be good on average. So far it worked.
That's still not a DDoS, as its missing the Distributed element. Almost by definition if you can block all requests at source its not a DDoS, its just a regular old DoS.