MADRID—Months after Catalan separatists made an all-out push for independence that triggered Spain’s worst political crisis in decades, Catalonia’s secessionist movement remains in disarray, with its leadership fractured and support for the cause falling among locals.

In a dramatic end to a sobering week for Catalonia’s pro-independence forces, former Catalan leader Carles Puigdemont was detained on Sunday by German police who were acting on an international arrest warrant issued by a Spanish judge. The judge wants to force...