Eject a villager from a minecart to the left

Solution 1:

From the wiki (emphasis mine):

Players can exit the minecart by pressing sneak, however if there is only one block of headroom above the cart, the player takes 1♥ of suffocation damage. When a minecart is exited, it searches for an adjacent or diagonal 2×2×3 area to put the player 1.5 blocks away from the cart. If none are available, the player is deposited directly above the minecart. Consequently, the player falls inside the minecart.

While this does specify this for the "player", this is also true of the mobs, and in this case, the villagers.

So, what you can do (as Fabian Röling suggested) you can use transparent blocks to dictate the only "available" space to deposit the villager, to block off any other "valid" spaces (i.e. forcing the villager to be ejected to the left of the track).

A player or mob riding in a minecart does not collide with or suffocate in any transparent blocks, but suffocates inside opaque blocks.

Source