The question I have with all of these theories, is would it be believable for a society which has developed with ready access to Soul casting to have developed them, we know they can soul cast large and complex structures into different, materials eg: barracks from air, and that they sometimes carve things from wood before transforming them to stone eg: the hand rail in the Palanaeum (chapter 33) but do they have the understanding of physics necessary to construct a proper arch that would support it self, or would they just be able to soulcast arches from wood.
I think the main reason they haven't soulcast bridges and fortresses as forward defences is the restrictions on their use of soulcasting, done only at night under strict guard, they would probably have to have guards down in the chasms to make sure there were no spies in the bottoms of them, as well as having secure control of all surrounding plateaus, it would be extraordinarily expensive to field that many guards over night, especially since they don't even know which direction to advance in, they dont know where the Parshendi base is, they wouldn't want to charge out and get cut off after all, that would be extremely expensive, and cost them troops and soulcasters as well. Considering that most of the High Princes don't care about the war all that much its hardly surprising that no-one has considered these options. We might see some scouting in force now that Dalinar is taking complete control of the war effort but even so the cost and difficulty of building forward bases makes the reward negligible, until they know exactly where the Parshendi base is the manoeuvrability of temporary bridges is vastly superior strategically and much cheaper. After they find the Parshendi base a need for secure supply lines and the likelihood of some kind of siege developing would probably prompt them to make more permanent bridges and forward forts, at least, I would were I running the army, but they might have very different methods of waging that kind of war than we do.