Chris,
I can not comment on the seeds having been in the onion over night, I simply do not know. Given the thoroughness by which the Bhaji were made and that the seeds were resting on top, I would guess they were not left in over night.
Remember the onion was only prepared the day before because of the time it takes to actually process them. If it helps, the onion looked quite fresh and unchanged. I doubt there is any real change overnight and if so, only to the top most layers.
I have estimated the amount of potato used. The picture showing the chopped potato was purposely done to show the volume. At best its 0.5 to 1% of the mix. For an idea of how much potato was used, you can see a round washing up bowl to the right of the chopping board, this was full of large whole potato on one layer.
What may not be instantly obvious, is that alot of this is done by eye. Nothing is really measured apart from a handful of this a bowlful of that. To you and me, this is confusing as we intend only to make a small part of this. We need to be accurate with our measurements. But on this scale, any minor changes in the amounts of ingredient is simply insignificant to the finish item.