All it takes is ONE part to be broken or missing and allow the shoes to contact the drums a little crooked or out of place and they WILL lock up. As you have confirmed. Just like
@Jadaharabi first said. He has a tons of experience on these things and there's no substitute for it. People who think they can diagnose the world from their computer, slide rules, formulas and calculators are usually wrong. Glad you got it fixed. Brakes are nothing to mess around with. Can we see some pictures of the car?