I think the question is about who spends edge first, attacker or defender, pre roll and post roll. The CRB doesn't say.
2. and then you you both decide to reroll 3 dice (before any dice are actually rolled)
I don't play that way - declaring your intention to use post-roll edge before any dice are rolled is too punishing; but even so, the question being asked is "who decides first", and "can they amend it"
Here's how I play. Post roll is easiest - Each side just asks themselves if they're happy with the outcome. If someone isn't, they spend edge and make the change. They're locked into that result and can't amend it further. Then the other side gets to choose whether to also spend edge.
Pre-roll, much the same way, but you have to pick a side to ask about edge spend first. Just go with the attacker. Attacker decides if they're going to spend edge, then the defender. But if the attacker says No and the defender says Yes, then the attacker can amend and the defender is locked in.