Are you SURE that it takes direction and accessibility into account when placing invaders?
What do you mean by accessibility? If an adventurer and his army of rescued children can walk over the mountains, why would a bunch of goblins not be able to? 40.x goblins found it tricky, sure, but that was a bug.
Yes, they actually move around on the world map, none of the armies in dwarf fortress "teleport" anywhere, they can move in the same places adventurers can, which is why you don't get invasions if you embark on a secluded island.
There are several different possibilities here that all boil down to the
resolution (or "abstraction level") of the checks (an interesting question is how it checks that and when/if those flags get updated after the player changes something).
For example, consider a square of 9 world map tiles adjacent to each other, so they are indeed all accessible on that level of abstraction.
However let's assume if you "zoom in", only one tile actually permits physical movement to the middle world map tile (a narrow bridge for example) - the rest is blocked.
For the sake of the argument, let that entrance be in the north (Like a 'U').
So now, your fortress is in that middle tile and the invaders are to the south.
If the game only checks general accessibility, and then "teleports" the armies to your site, they would appear on the south edge of your fortress map. (old behavior IIRC)
If the game checks on a higher resolution, they would appear on the north edge of your embark area. ("direction and accessibility").
I'm wondering, what is the actual resolution of those checks: How does the game deal with different resolutions of the same obstacle? i.e. a U-wall/moat 1. on your embark area, 2. a U-shaped obstacle on the "travel map", 3. a U-shaped obstacle on world map scale (a peninsula for example).
In other words, does an army (or anyone else) have to physically move on each tile, or do they only have to physically move on the "travel map"? Can they then move through travel map tiles from west to east, even if a bunch of them are technically only accessible from the north (but consequently marked as 'accessible')?