I have too much invested in my XO deraileur, shifters and PG990 cassette to want to go to 10 speed now, but I do need a new crankset and want to get the latest XTR, but everything I find is 10 speed compatible. Seriously, I can use that with a 9 speed cassette and chain, right?

Also, having a Hollowtech II, pressfit bottom bracket, I don't need to be concerned about the outboard bearing bottom bracket that comes with right? I'll just discard that or sell it on ebay. Spindle width is still the same, right?