5: What is special about bridge models that is different from ship models?
They use same principles but there's more complexity to bridges as you're working with many more different elements.
4: What issues exist with more modern exporters that prevent bridges from working properly?
I believe this is what ultimately stalled BC bridge modding. You can't export a new working bridge other than via Max 3.1. The export functionality was never updated in later versions. You can add/delete set elements in Niftools to create a new bridge
. I experimented with this on a few trial bridges (I'll try and find some images). But I personally don't think it's ideal or as creative as the traditional method.