Regarding your questions about how and from where did the conditioned souls fall, your first question if someone has a relationship with Lord Krsna on Krsnaloka, does he ever fall down? The souls are endowed with minute independence as part of their nature and this minute independence may be utilized rightly or wrongly at any time, so there is always the chance of falling down by misuse of one's independence. But those who are firmly fixed up in devotional service to Krsna are making proper use of their independence and so they do not fall down.
Regarding your second question, have the conditioned souls ever seen Krsna? Were they with the Lord before being conditioned by the desire to lord it over material nature? Yes, the conditioned souls are parts and parcels of the Lord and thus they were with Krsna before being conditioned. Just as the child must have seen his father because the father places the child in the womb of the mother, similarly each soul has seen Krsna or the Supreme Father. But at that time the conditioned souls are resting in the condition called susupti which is exactly deep sleep without dream, or anesthetized state, therefore they do not remember being with Krsna when they wake up in the material world and become engaged in material affairs. I hope this will satisfy your questions.