I think there is no definitive answer here and you should consider every one and pick the best of all.
Why? This is common, when one talks about ground loops, return currents, or just proper grounding to get tons of replies that come up with completely different conclusions. They all have good technical arguments. All will explain that they have done so or so thousands of time and that it solved the problem all the time... Well, probably they are right and this is true.
Proper grounding is dependent of the application. Good practices in analog design are disastrous in high speed digital. RF grounind has nothing to do with high power DC grounding. Then comes the argument of safety ground (Earth) and ESD/EMC/EMI issues.
For instance, there is a good book about the subject "Electromagnetic Compatibility Engineering" by Henry Ott, which is very good. But good at what? The book is oriented towards EMC/EMI. It doesn't address (or at least not thoroughly) proper grounding for audio for instance.
This doesn't answer your problem alone, but combined with the other answer you will get, it may help a little bit.
In order to contribute more to the final answer I would say that: What defines if your solution is correct or not? In your case it's if the amount of noise you get at the input of your ADC, relative to AGND at the ADC pin, is lower than the resolution of that ADC. Thus we need to know the amplitude of your input signal and the requirement in term of noise.