1. The RX diversity : the MS sends a signal through 1 antenna. The BTS receives it on 2 antennas. Each antenna receives a signal which is a little bit different.
2. see 3.
3. Yes, it’s just naming convention. Both signals (main and div) have an equal importance when they are recombined. It can happen than the DIV RX is better than the MAIN RX. In this case, the DIV RX will be the greatest part of the recombined signal.
(see combining techniques in wikipedia : en.wikipedia.org/wiki/Diversity_combining)