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Wy they mold depends on how and when they were picked, the weather etc. One trick to make them last a little longer is to wash them on the outside with bleach water before using. Kill as much of the little bacteria and mold spores as you can. Once they are carved they go down hill pretty quickly, especially indoors.
Big Jack-O'Lantern type pumpkins are usually not good eating: everything it had went into being big, not sweet.

I just finished a roasted pumpkin in pumpkin-cranberry bread. I had to use a jack-o-lantern pumpkin, so I got a smaller one. After roasting, I pureed it with a blender. It worked. Roasting definitely helps concentrate the flavors.

I usually carve pumpkins only a day or two before Halloween. Outside it will keep a few days, but no way it will inside. How long do cut vegetables look good on the counter?

You do need a pumpkin variety intended for pies, and the one you're most likely to find is usually no more than 5 pounds. Have used actual pumpkins that I've grown. They tend to be stringier than squashes, and the flavor is more intense. I really prefer squashes but sometimes give in to the romanticism of eating pumpkins from scratch. As you said, they sound good. If you do it plan on processing in processor or blender to deal with stringiness.

I'm glad you brought this up. As much as I enjoy making jack-o-lanterns, I always feel like I'm wasting food. I have made pumpkin pies from a large pumpkin - the kind you'd buy for a jack-o-lantern. I cut the pumpkin into quarters, removed the seeds and steamed it. I then allowed it to cool before peeling and running through the food processor. I thought it was a tasty, natural alternative to canned pumpkin that recipes usually call for. I have found steaming is better than boiling which tends to water log the pumpkin.