In the normal course of events, the mode is killed or sacrificed, things happen, and then it leaves play. Dismount resolves first, because it happens "When this model would be killed or sacrificed" and is presenting "When this model would be killed or sacrificed, [do something else] instead of killing or sacrificing it." "If this model leaves play" is a condition which happens while you're resolving the process of damaging or killing or sacrificing a model. If you have a rule changing the process of killing or sacrificing the model (which Dismount does), the rule that changes the process has to take precedence. So you resolve Dismount, move everything from [Mounted McCabe] to Dismounted McCabe, and then the rule on the Jack Daw upgrades isn't relevant because they're all now on a model that isn't leaving play. With this in mind, went back and re-read Dismount's exact procedure and timing, and it all makes perfect sense now. Much obliged all, for helping!