If I roll a crit defense on the green die, and negate all of my opponent's successes, can they play switches to gain more successes on that roll or are they preemptively canceled?
I'd have to glance back at the book, but I think the crit is part of the last step, after switches have been done. Which also means that your opponent could go ahead and spend successes for other effects first. For example the switches on the rippers and heavy shotgun that damage adjacent enemies.
No, it happens before switches. But I'm not asking about that; I'm asking if you can gain successes using other effects (like using ! to gain successes, which is a relatively common switch).
You are right, sorry. In that case I would say that all * are cancelled at step 2 or 3, depending if it is an enemy or character. Then, when spending switches, there are no * available from the dice, but switches can still be spent with whatever other symbols still remain. Which means that you might be able to spend !! for a *.
I would read the steps in order. Since the crit step is at 2 & 3, that's when I would do it. Then, when you get to the switches, that's already been done.
I had time to check. It is on page 12 of the rules. There are the sequences for managing Simple and FtF rolls. They are steps, like in Aristeia!. You complete one step and go to the following. Criticals are used on step 2-5, switches are activated at step 6-7. Any "added" symbol (Like ) should be treated as a separate dice rolled.