Performing Salaah between pillars

Q: In a Masjid, a row is usually left empty after every few rows, as it has several pillars.

1. If it is left empty, is the Salah of those behind it valid?

2. Is Salah valid between the pillars?

3. Once, people prayed in that row to the right of a pillar, but to the left of the pillar the row was empty. Was the Salah of those who prayed to the right of the pillar valid?

Killing a murtad

Q: I want to know the ruling for people leaving Islam. Should we kill them? Also please tell me what Quran says about a murtad? Why do people leave Islam eventhough they know the truth?

Returning an item to it's rightful owner

Q: If a person leaves something in someone's house and then he forgets about the thing and the other person also does not give his thing back to the person who owns it and now the owner of the thing is not in contact with the person. Someone told the person that he has to give money to some Islamic center if he can't return it. What is the solution that would be better?