Water on top of slowly receding magma and obsidian-based floor formations - flow control?

The question is about the interaction of magma and water, and ensuing changes in topography. The DF wiki says that when magma is mixed with water you end up with steam and some obsidian from the interaction; it also states that volcanoes will always refill themselves.

Context

What you have here is a volcano at ground level and I didn't pay attention when digging the moat around the fortress entrance, so there was a cave-in and I decided to channel the river to the volcano instead, dedicating the fortress to geological "science". Since the dwarves deconstructed the trade depot twice when the elves came (next time I'm building an ashery next to the depot, blocks of coal and oak wood goblets), the wealth must have triggered a megabeast attack so our tribute was in blood. The idea to build an ever-lasting sauna platform atop the volcano came to mind, but safety first, so we dug a channel to bring the joy of magma to the caves below.

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To summarize, you have the river going into the volcano and the volcano emptying itself some 15 layers below into the caves. The water is on top of slowly receding magma. My issue is whether a floor will entirely form itself out. As you can see top left or center only a few tiles seem to connect to the underlying magma at this point and this is barely 3-4 levels below the surface.

I'm trying to gain insight and analyze possible outcomes.

• Should I increase the magma flow in the caves to prevent such a floor from forming or is it unavoidable?
• How many tiles of magma flow are required to "fend off" (stop floors from forming) a 3-tile(x2) water flow on top of it?
• Is it possible to control the process to steer how the topography gets shaped?
• Generally, had I not channeled the volcano into the caves, would a floor have formed immediately on the surface where the water meets the magma, or do you end up with unlimited steam; can you have a water flow that is little enough to not generate floors (only steam)?