Dealing with creature collision is very hard and much more complex than what you seem to think. What would happen if creatures collide in a corridor one wanting to go up and the other wanting to go down ? What will the other creatures will want to do ?
You can divide the tiles as much as you want, there is no solution. At most, you can try to add something smarter than the random offset I added when creatures are walking but that's it. Going further is an enormous work for a, IMHO, not that usefull feature.