Creating bridges (in modelling program) relatively easy - as many have done. Getting it ingame, and then to work as you'd expect a functional bridge, requires steps 2-7 as above. Is it rocket science? No. Is it difficult and time-consuming? Yes. But maybe we've all got a bit jaded here and actually what is needed is a comprehensive practical guide so people know exactly what does entail. I would say in brief:
1. Before even start, study existing bridges and scripting in detail. How they work, how scripted. Learn how to change textures and lightmaps so you can see how they affect bridge ingame.
2. If still determined, get a modelling program. Max is good as it allows you to import nifs of existing bridges, allowing you to study how constructed, use of meshes, textures, etc. Depending on time, skill and level of detail, this could take several weeks or several months.
3. Assuming you can build your bridge, now comes the first hurdle. It will only work if exported as a nif from Max 3.1. Nothing else. Later versions won't work. You can export from later versions into Max 3, but that's it.
4. Assuming you can export nif from Max 3, you now have a bridge that will appear ingame. But what about the crew?
That's another major problem. Unless you've built your bridge around an existing layout, you have to place crew yourself. That involves using Character Studio (2.2, I think), the skeletons included in the SDK, exporting crew (and other moving ojects) as animations, and the placement of dummy objects in the model itself. Sorry for lack of detail, but haven't cracked this one myself
4b. Alternative to above is to take an existing bridge script and physically move the crew via xyz axis (either in animation or script) to correct position. Not perfect, but an alternative.
5. If succeed in all the above, have scripts that work and appropriate permissions, you will have a bridge that can be used by BC community. But there's a lot of hurdles and hard work to reach that point.
Hope that helps

Good luck.