1. What is the reason in case of B+Y for gravity extension
To preserve the Green Hydraulic System integrity (FCOM 3.02.29).
Both slats and flaps are now being supplied by the GREEN System, the extra (Enormous) hydraulic load from operating the gear would not be prudent with BLUE and YELLOW out.
Should we limit crosswind to 15 knots for limitation reason due to 3 spoilers inoperative, and in case G+Y what is the reason to show landing distance in summary on wet&cont RW if it is prohibited
Please clarify this question. I
understand from what you wrote, that you are asking about a 15 kt limit, and a prohibition on "Wet" regarding G+Y, to which I am not aware of any restriction (QRH).
In case of G+B why should we do LG extension at 200 knots and how to do it if simult.
For improved controllability on a single elevator (FCOM 3.2.29)
Simulate? That's what SIMULATORS are for.