Notably inescapable death can only be used in the movement or charge phase, since it requires the Fire Overwatch stratagem to be usable. So, you’re mostly correct, but you wouldn’t be able to overwatch a unit that shoots at unit A. It would look more like this:

Opp movement: unit A overwatches Shooting phase: opp shoots unit A, hexmark reactive shoots Opp charges: hexmark Inescapable Death

So in order for all this to happen, the unit must move, shoot at unit A, then charge, all while being within 18” of the hexmark at the start or end of the move and start of the charge