Merchants are scared of my door guards, and I have to unassign them to let the merchants in.
Have you tried using wall grates and levers?
let me mangle the most important sentence i read here last week.. " its guaranteed that traders will spawn where you want them if that's the only way for them to travel that 3 wide path to the depot " ... four fun forts later i've see this work .. and fail unless you have a 3x3 spot next to the edge tile for the wagons to show up on (so no channeling down right next to the edge).
(hint, you can not remove the ramps on the edges, but you can put floors on them)
methinks you could use that for the wagons. give the diplomats some fast, multiple entry way access tunnels from 8 points along the edge and only the guys you want to come to your front door (you have a roof, right? <G>) will actually come there.
when i care, i use this to let diplos in:
..hhPHWW
..WWWW....
..hhPHWW
-- h's are throw away hatches that feed the building destroyers. i used to use one, but in my games with the new version had building destroyers actually walk over
one to destroy the pp behind the grate. now i'm using two trash hatches while keeping an eye out (you dont actually need them, but it beats having to rebuild pp's)
-- PH is a pressure plate hatch combo that does not trigger for citizens.
-- both PH's lead to the same .... path.
if you use these citizen only entrances as part of a wall or just below ground, make them be short and for peace of mind, have them all come out in a short tunnel with weapon traps and a bridge/hatch you can use to seal access. usually this goes one below ground, just above any staircase plumbing. incidentally, quite often the exits of all my defense related ditches, yes i give them exits, wind up adjecent to the speed entrances.
in the meantime, i'm going to put some captives out there behind bridges, sounds like a great idea!