I've never had an issue with DayZ Commander and Steam. Worked straight out of the box, so to speak. I don't know of anyone who had an issue with it either, and all of my friends have Arma2 via Steam. Out of my gaming group, we have around 14 people who play DayZ.

Just install it. At worst, you have to point DayZ Commander at your Arma2 steam folder.

DayZ Standalone for June is not confirmed. Also, it's only Alpha, and the standalone is a separate game. Which means a price will be involved. If you already have Arma2, just play DayZ as it's free until we get more information about the standalone.