Cults are about power, specifically for the leader of the cult. It's reasonable to assume that the mayor of the town and most of his cronies are cult members themselves, willfully working with the cult for mutual gain, or the cult has some kind of leverage over them.

Let the sorcerer get a mob, which while enroute to the cult to rescue their friends will be joined by the sheriff etc.

Meanwhile, the rest of the party will want to fight their way out or die trying, and it won't be fun for them if you railroad their actions in another direction. So, let them! The cultists won't kill them without orders (because maybe it's okay to kill these people, but maybe not, and none of them want to take the initiative and risk upsetting the leader)

The party successfully breaks free just as the sorcerer shows up with the mob lead by the sheriff who's on the cult's side. He then "legally" arrests the party for attacking the towns citizens. The civilian members of the mob are easily quelled and dispersed by the sheriff's men, leaving the party to fight a proper mini-boss battle (the sheriff and his men) OR let themselves be arrested (maybe by the suggestion of the NPC friend?) as a means of getting closer to the truth and whoever is REALLY in charge.

If they fight and win, they'll have a blast and earn their escape.

If they let themselves get arrested, you can advance the plot or have a sympathetic townsperson help them escape stealth fully.

Either way they should have a fun 🙂