Why didn't Goku and Chi Chi use the Dragon Balls to rescue the Ox King and his castle?
Your assumption about the dragon balls was wrong. They did indeed scatter in chapter 156, right after Shenlon brought everyone back to life. Also seeing the mini saga about the wedding-dress was anime filler to begin with, one could argue that it would be pretty lame to fill it with the gang just using the dragon balls.

Apart from that, it should be noted that getting the dragon balls wasn't as easy as it sounds at that time. Bulma did have the dragon radar, but she would not have been able to retrieve them all on her own as we had seen earlier in the series. And Goku couldn't help her either as he had promised to train at the sanctuary with God for three years. He was stuck there all the way to the tournament.

So when the fire actually happened, it would take Goku less time to get the fan, than to collect all dragon balls. So trying to get the fan first, would be the best choice of action. If he would fail, they could collect the dragon balls as a backup plan anyway.