[acdveins]

I would say as soon as someone declares an attack they lose the chance to reposition their figure. I don't know what the official ruling on this would be, but that is the way I would play it. My group plays very loosely though. If you forget to move a hero before the bonded squad, you move him later, you can reposition as long as you haven't attacked, etc.
And don't feel too bad about losing that free swipe from engagemet strike. It works more like a deterent for engaging the Nakitas/Arkmer than a free attack.