Grounding and Signal Integrity of my PCB Layout (ADC, SMPS, SD card, USB)
When it comes to noise, it would probably help to add a couple of 10uF ceramic capacitors in proximity of the opamps, as well as to the SD card. The SD Card will have quite fast transients during the R/W operations, which will transfer as noise along the 3V3. Additionally as you are measuring batteries, you can get rid of a lot of the noise in FW, as the changes in the values on the batteries are very slow.

EMC and ESD
 If I understand correctly, this is going to be a product, and therefore it should comply to local emission regulations.
The first things that I notice, are that you have no ESD protection on the SD card or the USB; I suggest adding some low capacitance TVS protection diodes on both power and data lines.

The USB traces should be routed as 90 Ohm differential pairs; if you do this correctly, you will reduce the chance of reflections on these lines, and therefore conducted emissions.
I suggest you download the excellent PCB Toolkit from saturnpcb.com, it is a great tool, which has one function that will calculate for you the track impedance. It is not as good as the Polar tools, but it is free, and gets you what you need.