leave them in the hatching container till the belly button is gone,cover them with a damp paper towel to prevent dehydration,usually they come out and bury themselves into the medium,then when they are ready surface and start looking for water.The yolk sac is very prone to infection or rupture if you put them in the water too soon.As for the other egg,if its clean and white just wait,there is no fixed time even for eggs of the same batch,tampering will result in tradgedy.you probably could put some acrflavine or similar in the water,but i would advise take it out paint the area with some dilute iodine etc after a day or so.