The best pattern is 9x9 with a water block at the center and for MCO a slab on the water block. If you don't put the slab, villager can fall in and they are stuck.

The pattern -> http://hpics.li/9c65d3f

We should also be able to define (with a wand like world edit) where exactly the center of the pattern is because it is sometimes hard to make them stay on a specific block (mood : ecstatic).

they also should put a chest and automatically put all they harvest at the end of day so we can make automatization.

If there is not enough space to finish the farm (missing dirt block or any block that is stucking them), we should be able to give them a pickaxe and dirt, then they could totally finish the farm.

Sounds good!