Thanks entirely to the valued support of the foundation for public boardposting and viewers like you - the main issues are at last hammered down and done.
The routes are working, my engineer camp for the allied footbridge is now functioning to my satisfaction, and a I squeezed in a few minor navigational node tweaks as always to iron out the pathing. I made a few adjustments to my aiscript, mainly to weed out some bad actiontests that were in there due to an oversight on my part.
The only issue that may need addressing at some point, of which I am readily aware is that on occassion, firefights can erupt on the footbridge, and when the bots enter their usual jump/strafe evasion tactic there are times when they fall into the frozen river and are savagely plunged into the deadly depths of hypothermia.
I'm still kicking around ideas for how to solve this, the most obvious of which is a quick path leading out of the water and back to the shoreline so that at the worst case if they manage to swim out they'll just be wounded rather than dead from the slow hurt trigger in the map at that location. If anyone has any fancy tricks up their sleeve as to how to make them a little less bouncy in that confined space - please do share.
EDIT: Also note I'm refining a duplicate route for the allied document steal objective from the spawn flag (since they do seem to use it alright when its in their possession) and in my current internal version (what I tenatively call 4a hehe) I've set the spawn flag objective to always active - though disabled by script if the bridge is destroyed. While I'm not even sure if the route could be disabled in addition to that, it shouldn't matter since they have to spawn there to use the route anyway - and disabling the alt_roams would shut off the other routes as well, which I don't want. I suppose I could duplicate the alt_roams, but again a waste of time since no spawn at that location means no route usage anyway.