Once cooked, rice should only be reheated once for food safety reasons. So once you stuff the pumpkin with the cooked rice and bake it you shouldn't reheat it again.
You can easily make it in advance. Just cook the rice and cool it quickly, mix with the other ingredients, stuff the pumpkin, wrap and keep it in the fridge for up to 3 days before baking.
You don't need to roast your pumpkin in advance. Just put it all together and keep it in the fridge until you need it, then bake and serve. If you roast the pumpkin in advance without the rice in it you risk it getting too soft and falling apart when you stuff it.
Hope that helps.

I added lentils because I try to incorporate some kind of beans in every meal. Came out great! Last year I brought to a Thanks Giving dinner with turkey eating friends. I had this with a couple sides and when when back for pumpkin seconds it was gone!

Wish I could find unsweetened.

I think sauteed eggplant, artichokes, squash, zucchini would all work. Chickpeas would be great too. And black beans. The filling is really flexible. As long as you keep the quantities roughly the same you can add whatever you like, customizing to suit your tastes. I hope that helps and that you enjoy it when you get a chance to try it!

The spinach just gets mixed into the bowl with the other filling ingredients. You don't need to cook it first.
Dried cranberries are best for this recipe. They add chewy texture and little bursts of sweetness. Fresh ones would be a bit tart and probably end up indistinguishable amongst the rice once it was all baked. They might also be a bit tart.