Mixing questions about stereo image.

Some of that "spatial" quality you're hearing (and losing in mono) is due out-of-phase tracks. It's a common aural trick but destroys mono compatibility. You'll have to decide if it's cool enough to sacrifice mono compatibility, but it sounds like you'd rather find a third solution.
 
Panoramic width is all about creating differences between left and right channels, ideally differences that when combined don't cancel any frequencies. EQ is a good place to start; boost and cut complementary bands on each side, e.g. boost the right at 2KHz, cut the left at 2KHz. A graphic equalizer, which normally has zero uses in a studio, is very handy for this.
 
Another trick is to use delays and reverb to generate L/R differences. Reverb by its nature spreads reflections across the panorama (many reverbs include a 0-200% "width" knob to control the dispersion distribution). You can emphasize that by (slightly) panning the reverb opposite the dry track's pan. Use Channel Tools rather than the pan slider for that.
 
Assuming your 5 vocal tracks are a lead and four harmonies, you can get away with effecting the harmony parts a lot more than the lead without sacrificing lyric intelligibility. I like to apply a chorus to background vocals and pan them 30-50% rather than hard-panning. It doesn't take a lot of this effect to noticeably widen the image while not sounding obviously "chorused".