On live sound when I've got a clear distinction between lead vocalist and backing vocalist(s), simply backing off the high-mid/treble (depending on pitch of voice) for the backing vocalists works pretty well. It preserves the basic pitch of their voices but loses some of the consonants, letting the lead vocalist cut through the mix regardless of how loud the backing vocalists are.

Re the chorus effect, the traditional guitar-type chorus is probably not what you want. Use a slight delay, but then also use a pitch-shifter to move it slightly up or down from the base note - not far enough that it sounds out of tune, but just enough that it's distinguishable as a different voice. I haven't yet found a pitch-shifter VST which will put a random variation on the pitch-shift to make this more realistic, but I'm sure there's one out there somewhere.

Graham.