It’s perfectly fine to choose who dies as long as the players are aware that you WILL be doing this if they lose the conflict. I would simply choose death for the mouse that makes the most thematic sense.
BUT, the intents for their opponents damn well better be “They want to kill the mice!” The intent you listed will not allow a mouse to die, if I read the rulebook correctly.
I try and make it obvious when framing the conflict: If the patrol’s plan is for Saxon to lure the badger across the scent border (chase conflict), then he’s the one risking death. Sometimes I just pick a mouse and that’s the animal’s conflict goal (Eat Liem). Usually, the fiction and narration make it obvious who is most liable to take one for the team, but it never hurts to have it spelled out in each side’s goals.
The example I gave was just for making it explicit to the players who the targets were. The tenderpaw character wasn’t at personal risk but could still be injured should the weasel’s intents come to pass.