Grounding in mixed-signal systems demystified
"Demystified" with small print disclaimers applied :)
Your goal is to contain AC and DC current loops so that the voltage drops they develop across the ground plane impedances aren't adding up to the effective input voltages fed to various inputs (of the gain stages, ADCs, output amps, etc.)
Adding slots or otherwise physically separating the ground planes is just one of many means to achieve such containment.
Any work in this area should first quantify to an extent what sort of currents, frequencies, and parasitic impedances we have to deal with. Then estimate the worst-case "noise" or unwanted coupling from those current loops to sensitive nodes. Then decide how to route those currents so that they don't cause problems. Such routing and containment may involve adding cuts to ground planes, relocating components, overlapping current loops so the fields they produce cancel out (even if partially), etc.
should digital ground plane be the ONLY one connected back to PSU ground
Usually, but it really depends on what other connections there are. A system block diagram depicting all external I/O to your system (power, digital and analog) is necessary to provide any guidance in this respect.
The starting point is always a single uninterrupted ground plane and well identified current loops that are arranged to circulate locally, affecting only small areas of the reference planes (be it ground or power).
You may often find that the performance is well within the requirements (which you didn't state, BTW).
Such a plane necessarily has the lowest bulk impedance between any two points. As you start removing copper, you trade off impedance for potential containment and isolation of interfering signals. As with all trade-offs, if you can avoid it, so much the better.
Should the analog LDO (i.e. A_VDD power supply) be placed over the digital ground plane area? Over the analog ground plane area? Or neither, and with a trace from one of the above, but with no ground pour below?
The LDO can straddle the two planes. The connection between the two can be made at the ADC, or at the LDO, or somewhere in-between. The LDO should be then very close to the ADC.
With a 2-layer board, such designs become "interesting" and you should not expect the first version to work as well as it ultimately could. In absence of sophisticated modeling tools and battle-hardened intuition, you'll have to experiment and then connect the experimental results to theory, i.e. understand what parasitic impedances play a role and why.
You'll want to set yourself up for precision differential voltage measurements, i.e. build (or buy) a differential wideband preamplifier you can use to measure small voltage differences between nominally "connected" points in your circuit. The preamp bandwidth needs to be solid - 100MHz would be nice, 10MHz would be better than nothing.