I have shot D3300 with 35mm F1.8DX, and upto ISO 800 if works fine, though ISO 400 is much cleaner. So you can safely use 50mm F1.8 + D750 as long as you can get the full width from where ever you are positioned.
Given sufficient light the 24-70 will track and focus perfectly, but if the light...
I have been using NX-D since its introduction, and have never faced this, except when editing JPEG files. In fact all the sliders work whether one or a whole lot of images are selected. Here are screen shots
Actually in good natural light a good cell phone will give excellent images, good enough for A4 prints.
In case you are bent on using a DSLR, I would suggest that you rent a D3300 at least a week before and practice till the wedding. Alternatively you can buy a D3300 with a kit lens and a 35mm...