Regarding the dwarven x-ray: it will only reveal four possible states of otherwise unrevealed tiles (full, empty, magma, water). And since empty spaces create Interesting Things only after actual reveal, by mining or cave-in, this is still a surprise. But you know there is a surprise there.
Regarding fool-proofness: there is one or two catches. First, even 2/7 water can create a current strong enough to push the miner, who can then obsidianise himself. So the more fool-proof method would be to reveal magma tiles, then evacuate miners, and then flood the level. This allows for incursion of magma crabs, but they somehow die off before I come to this point (low reqional population), but it can't be disregarded. Alternatively, wait till most or all tiles are 1/7, this is safe, no water pushes.
Second, sometimes the Interesting Thing with funny syndrome is damaged and stuck in the single tile hole, so you cannot shoot it (because all bolts stop on the tile nearby above, depends on the angle, but game allows for impossible targeting, which only wastes bolts and time, and you may stumble unto it). Then you have to build some cave-in structure above, but this can be tricky and need more headroom, because dwarves start panicking when they see the Interesting Thing.
As a side note, when I first discovered a 3x3 gemstone/obsidian thingy in the caverns, I built a cave in structure on top of it, then I built a big sarcophagus around it, Chernobyl style, so when the cave-in happened, whatever inside would be still safely enclosed, and could be dealt with another cavein.... Wasn't necessary, because apparently they don't have anything interesting at this point of fortress life.