The obsession with wide bars does my head in, personally I think it should be about fit for your shoulder width, as the wider you go the more you need the bend to be to not stress your wrists
I ride with 600mm ones and consider them to be wide
To me it is a bit of a fad, wider bars have benefits to a point
Sort of agree (no idea about XC racing but I'm guessing the principle hold across the board)
As wide or as narrow as is comfortable. Try them out do some rides and see, the worst that can happen is you decide they are too wide and either cut them or replace them.
I am at 760 & 745 which feels right for my shoulders which are quite broad. Personally I reckon narrow bars is a fad that is coming to an end...