In Orach Chaim Siman 123 Saif 2, SK 8, the Mishna Berura brings two opinions regarding the amount of steps needed before one has to say Kedusha. The Kesef Mishna holds that after one's three steps back at the end of Shmoneh Esreh, he can stand there and recite Kedusha there. The Mishna Berura then brings a Magen Avraham who says that there is an opinion that you need 6 steps and therefore one would have to walk forward to say Kedusha.
My question is two part: 1) Where does this opinion of six steps come from? What's the reason for 6 steps before Kedusha? 2) Regarding that answer, would it follow that this whole discussion only follows an Amida that has Kedusha but a Maariv Amida would not have this six steps idea?
Please cite sources.