Question 2 - I would prefer "How much information did the police get by interrogating/when they interrogated Alex's friends?" We don't expect to get information after interrogating someone.
Question 3 - Change "there" to "in" and it's OK. I'm having trouble understanding why the police would be able to ask more difficult questions just because the friends' wives were in the room.
Question 4 - It's hard to say whether it was because it wasn't right, or because Felix thought it wasn't right. To be more challenging though, you could ask a trick question - "Why did Felix accept Adam's suggestion?" Then your students have to say "He didn't. He rejected it because it was wrong" or similar.