The membrillo just didn't behave. All I can imagine is that there wasn't enough pectin in the fruit - perhaps it wasn't ripe enough. The quince paste has caramelized to a deep dark brown, it tastes great but it's not membrillo. It has the rich, dark, sweet texture of my favorite apple butter from cider mills in Ohio and Michigan -- most apple butter I find these days is just slightly thicker than apple sauce.
Tonight, I heated up a batch and put it into 4 oz jars, and labelled it:
10 4 oz jars
I'm going heat up the rest of the quince paste and add pectin to see if that helps it out. I still have about 8 cups of quince pulp in the refrigerator and I'm going to add a shitload of pectin to it to make it set very hard. I will have membrillo, dammit!