Can Bill do this after both players have activated switches, or does the player controlling Bill have to do it during his/her Switches phase? For Example, Mushashi moves up to Bill and uses Ken no Sen. As the active player, Mushashi makes Bill switch first. Bill doesn't use any switches. Then Mushashi changes all his blocks to successes. Now that Bill can get damage, can Bill use Dead Man's Hand now to discard a Tactic?
Since it is already evening in Spain, I will attempt an answer. Dead Man's Hand states that once per Combat roll during the Switches step you may discard a Tactic to gain an extra Success. As you have stated the active player can determine who does switches first. Somewhere it states that all of a given player's switches happen together and then the other player gets the chance to activate switches. So my interpretation would be that Bill cannot try to use Dead Man's Hand after he passed on switches when Mushashi gave him first opportunity. I found the section of the Reference Guide that supports that all switches must be activated together. • The active player decides which player gets to activate Switches first. • Each player has only one chance per Roll to activate all their Switches, one after the other. You cannot activate Switches after the turn has ended. •
The thing is that Dead Man's Hand isn't a switch. It's an automatic ability that occurs during the switches phase, but it isn't a switch itself, so I'm not sure whether it gets included.
Wild Bill can activate Dead Man's Hand at any time during Switches Step. So he can do it right at the beginning, before/after his Switches or before/after his rival's Switches. Usually he waits till end of the step to decide if he wants to do (that's the advantage: he hasn't to waste a Tactic uselessly).
