Decoupling of A/D converter
I almost always place the decoupling caps immediately under the device for which they are decoupling (though if you can fit the 100 nf on the top side, that's fine).

Generally, you want 1 via per power pin, and you want to place the via as close to the pin as possible, and the 100 nF cap as close to the vias as possible. Also, make the vias as large as will fit. This devices does not have a exposed pad on the bottom, which makes things easier.

The bottom layer is then used for power routing, and the top is for signals.

Putting the decoupling components directly under the part is generally only a problem when you are doing manufacturing, because boards with all components on the top are cheaper than boards with components on both sides.