A. In an ideal environment speaker spacing is set by listener distance. You're trying to achieve a wide sound stage without opening up a hole in the middle. An equalateral triangle is a nice starting put for most speakers (when the speakers are 8' apart, you'd be sitting 8' from each which puts you 7' from a line drawn through the tweeters). You might space them closer together if you need to get them away from the side walls.
B. The greater of 4' or half the distance to the listener is a nice starting point. More is better, but changes in distance will interact with room modes differently.
C. Speakers are engineered for a specific placement and will have a surplus of bass and lower midrange when placed near more room boundaries than they were designed for. Placing speakers designed for use away from walls near one wall will give your singers chest colds and make the bass boomy and near two walls (a corner) the effect will be much worse.
B. The greater of 4' or half the distance to the listener is a nice starting point. More is better, but changes in distance will interact with room modes differently.
C. Speakers are engineered for a specific placement and will have a surplus of bass and lower midrange when placed near more room boundaries than they were designed for. Placing speakers designed for use away from walls near one wall will give your singers chest colds and make the bass boomy and near two walls (a corner) the effect will be much worse.