Beautifully designed article. Very cool images and diagrams.
> We want rocks in contact with water,
But there is ice compressed between the water column and the xenoceanic bottom, ok. I see the problem.
What about rocks in the surface of the water?. Some volcanic rocks can float.
> What about rocks in the surface of the water?. Some volcanic rocks can float.
This is... not something you lay plans on. Even on Earth, these rocks are formed at surface pressure, just barely float, and don't float for long (I've tried). If they form in the deep ocean, I'm guessing the pressure isn't going to let the bubbles be big enough to create bouyancy at all. Either way they would be super rare.
Just some thoughts
1) Animals can synthesize "rocks" with hard materials that will sink naturally, but that are buoyant while their owners are alive. This means that we can expect a thin layer of a fake "rocky" bottom in form of Thanatocenose just in the surface of the second layer of ice. Bony or Shelly bottom would be a better way to define it.
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2) If those hypothetical mineral resources are rare, organisms would be hard pressed by evolution to start collecting them.
Some animals collect pebbles to make pods. If the upper ice layer was created after the lower ice layer we may find aliens dressed on a meteorite's cape. This meteorites don't necessarily need to sink if included in the body of an organism. The Earth developed similar solutions that can be colonial, like Sabellaria worms, or individual, like caddisfly larvae.
Fill this pods with organisms able to extract the energy of chemical links in those rocks, and you have food and refuge in the same packet. Go a little further and you developed luminescence. We can't guarantee that this life evolved eyes, but bioluminescence is the most popular way to communicate among life beings on earth so if aliens didn't evolved eyes the alternative solutions will became very bizarre and unexpected really fast.
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3) Maybe we will not need to dig all the way down to find life. Biocenosis will fit every possible niche, so is 100% guaranteed that the transition "upper ice layer" -> "liquid surface" will be colonized by life.
First because it poses a topological advantage (halves the directions of possible attacks by predators, and allows predators to ambush and hide in irregular ice).
Evolution later would select organisms that burrow up in the ice layer. Burrows are a safer place to rest and nest than just a surface of ice, so the trait would be selected fast by evolutionary pressure. We can expect tunnels shaped as and 'A' populated by filterers also. They will create currents between an entry and an exit to use the energy more efficiently
So we may suspect the presence of life if we can detect indirectly a thin layer in the ice/water frontier that: a) is less dense than ice, but more dense than liquid or gas bubbles, and b) is structurally different than what physics would predict for the formation and extension of ice cracks.
And if there aren't any alterations in this boundary, it may be no life at all, or life may be very rare.
I guess I'm now officially old and cranky, as planetoids receiving glow-ups was just a bit much as a term.