Substitution for fresh corn

The best substitution would be a a simple tube pan.

Besides the unique appearance, bundt and tube pans are used for particularly dense and moist batters. They might be used for a cake that might dry out on the outside before the center cooks through.

If you don't have a tube pan, then there are a couple alternatives. You can use two loaf pans, this should approximate the bundt pan. Another alternative is to use a deep circular cake pan with a ceramic ramekin in the center of it to create the hole. With either method you may need to adjust your cook time a bit depending on your results. (likely 25F lower and baked for a little longer) It also might simply just work without changes.

You can sort of substitute, yes. Bear in mind that cornbread has a far lower gluten content than wheat breads (zero gluten if there's no wheat flour in the recipe), and thus will not hold together quite as well. So you may wish to use slightly less of the custard base.

In addition, I've found that cornbread doesn't generally make a particularly nice sweet pudding; it's much better in a savoury application. So perhaps throw in some jalapeno and some goat cheese.