Hint: think of re-spawning in video game terms. This isn't a new red slime being born when the characters enter the room again, rather its the dungeon "recycling" its defensive tools. This means that if one half of the pool is filled with a dead slime and someone enters that slime's spawn point its remains will be reclaimed by the dungeon and respawned for the character entering its room. Thus, the dungeon cannot be solved by backtracking.