You can minimize trader deaths quite a lot with a bit of planning by exploiting their behavior. Find a flat space at the edge of the map. You'll need about 7x16. Build a depot as close to the edge as possible and wall it in as much as it will allow with a 1-wide path to the edge. Depots can't be closer than 10 tiles to the edge, walls closer than 5. For the last 5 tiles, build 2 raising drawbridges that extends your walls to the map edge, hook them to a lever and raise them. Since traders will only spawn where they can path to the depot, they'll only spawn in the one edge tile that you allowed. You'll occasionally get an invader spawn there as well, but it shouldn't be too often. It should look like this:
+ wall
D depot
B bridge
+++++++
+DDDDD+
+DDDDD+++++BBBBB
+DDDDD
+DDDDD+++++BBBBB
+DDDDD+
+++++++
You can also make the approach 3-wide to allow wagons. Put an extra raising bridge against the depot between it and the map edge and turn the back wall into a bridge, and then you have an airlock, can allow traders in, pull 2 levers, and seal them off to the inside of your fort.
The liaison will be more of a problem as they spawn as invaders do and path to your dining room. Protecting the liaison is much harder as there's no behavior there to exploit, and they trigger traps just like gobbos. Lots of traps there. I play almost exclusively no-immigrant forts, and I find the best solution is a dedicated trader entrance from one edge of the map and then 3 invader entrances from each of the other 3 all leading to a central trade zone. The 3 invader entrances are all trapped very near the center of the map so there's less hauling to do of drops, and the trade depot is there in the center. One invader entrance can be sealed off to be cleaned out and the good easily brought to the depot, and just rotate as needed. With 3 entrances, the liaison has better odds of not getting clobbered on the way in.