I have 2 D800s and unfortunately they only work in studio....

The two guys by the Christmas tree could be one of the most difficult autofocus subjects I've ever seen because of the lights on tree and the possibility of camera movement because of the low lighting and slower shutter speed. I find that's AF–c seems to handle this situation a little bit better but from what you're describing, switching to manual focus might be your best alternative in those long-distance confused background shots. The D800s biggest issue without a doubt is too many megapixels for hand holding in marginally lit environments. I'll bet if you are on a tripod or a monopod when you shot this would have had a lot better luck. Your initial assessment of the D800 being a studio camera in many ways is completely correct, I am not sure if I shot weddings I would use this camera at all except for the bride portraits. Your approach of getting a D3s is the correct one I think.