Yes, prayers can help the dead. I've prayed for my mother, wishing peace and happiness for her now that she is in a better place.

You mentioned that your dreams vary depending on what position you sleep in. Years ago, I read of a study that was done on this subject. Researchers waited until volunteers were in REM sleep and then would do something like put light pressure on the volunteers' arms. It was never enough pressure to wake them. Then after applying pressure, the researchers would then wake up the volunteers and ask them what they were dreaming about. The study found that depending on where the pressure was applied affected what the volunteers dreamed about. I forget the detailed outcome but it did prove that how one sleeps does affect dreams.

To be honest, when I've had a lucid dream, I've never taken notice of my sleep position. However, I do know that lucid dreaming, depending on the circumstances, can be helpful. For example, with recurring dreams, a lucid dream can keep the dream from coming back. Once I read of a woman who had a recurring dream where a large, burly, menacing man was chasing her. So one time when she was having the dream and it turned lucid, instead of waking up, she confronted the man. She stopped running, faced him and asked him why he was chasing her. He said "will you marry me?" She said "yes". After that, she never had the dream again.