ok he has a good idea u know, he may have found out how to let u have a tunnel.
1.) make a building that looks like a opening to a tunnel, and then make tunnel peices. put um all next eachother and all and have them have no collision size. now, i mean the tunnel has no collision size
2.) set pathing blockers all around the outsides of the tunnel (regular+ and air)
3.) make sure that 1. ) it is somewhat below groun (drop a level), and make sure air can fit beneath it, otherwise it would look bad, with birds flying through the top.
this will allow ur units to walk through and all, with no interference
for visibility, u might wanns have a region that once enters changes camera angle to a angle that is like 1st person so u can se ur unit (like a tank waz , non gps mode)
i dont know if this would work but its worth a try