1. because since most channels are mono, it makes no sense sending in stereo a mono signal. if you want to send in stereo, use 2 fx sends.
the fx returns are stereo because most external effects work in stereo even with a mono input (double delays, stereo reverbs etc...)
2. in a live situation, the main output goes to the PA system, the ctrl room is for the engineer's use to hear better single channels without compromising the main output.
3. i don't understand the question. please rephrase...
4. so then the fx level can be indipendent from the channel level.