oops! You are right. I meant:
... so evilcorp.org cookies for evilcorp.org images on A.org will not be set for evilcorp.org images on B.org.
Can you edit your post?
Probably not. HN only allows editing comments for a limited amount of time. In one post I found from a couple of years back, someone said that the edit window is two hours. I think perhaps also that as soon as someone has replied to a comment it will not be possible to edit that comment any longer but I'm not sure about that.
I'm not sure this description is as clear as you intended.