Maybe this helps you:
for your second question, download the Renegade Tools from
http://www.thundermods.net/downloads...download&id=18 , they contain a w3d viewer which shows your models in the original w3d format. It is also useful if you want to view some of the models EA created. This is especially important if you want to animate something since you can learn from those models that are shipped with Generals.
for your third question, as fas as I know, there is a threshold for the vertices to be welded. That means if your vertices are to far apart from each other they can not be welded. But actually you should explain it more in detail to help me understand your problem. Another solution could be to completely redo this part of the model.
for your first question I am unsure whether this helps. You know that faces that you create always point in ONE direction? That means that if you create a face, it will only be visible from one point of view. If you want to see these faces from two sides you have to create another face or select "2side" in the w3d export options this causes the face to be visible from two sides.
Hope that helped, HEXman