To Working, re: the crock pot bag liners, it’s important to lift up the top edge of the bag so it is not in contact with the hot crockpot, esp. when it’s on high. You may be able to fold it down when it’s on low/warm. That may be why it melted.
My friend was concerned about the chemicals in the plastic leaching into the food for so many hours, so she wets some parchment paper, and lines the crock with that. I did that for a few weeks, but it always leaked somewhat, and the mess wasn’t worth it to me. So, I went back to the bags. (I consider it a “sholom bayis” solution bec. I used to ask my hubby to clean it, and he didn’t want to, so..)