The findings below are probably already known, but I haven't found most of them.
I've found that you sometimes can dig Up/Down or Up staircases in SMR or Slade from above provided the area hasn't been revealed and you build (not dig) an Up/Down staircase above to give the miner somewhere to stand. Building a Down staircase above revealed the tile below in all my attempts, but a built Up/Down staircase did not in any case (I tried to build both on a magma flow directly as well as a magma flow converted into sand by building a floor and removing it before building the stair). However, digging the Up or Up/Down staircase is a hit-and-miss proposition that sometimes results in a staircase and sometimes a revealed tile and an "inappropriate dig square" message. I haven't discovered any logic as to when the digging will result in a staircase and when it just results in a revealed tile.
Successfully digging an Up/Down staircase reveals the tile below the staircase and thus blocks further digging, as does conversion of an Up staircase into an Up/Down one.
I've been able to use the staircase method to dig down into rock covered SMR in one instance (the first one failed with a revealed tile an an abortion message), so SMR digging is not tied to the (former) existence of a magma flow on top.
Furthermore, every time I've been able to dig out a Slade tile with either an Up or an Up/Down staircase a Slade boulder has been produced, so Slade mining seems to operate similarly to candy mining (except digging a staircase in candy results in a staircase made out of candy and no boulder, while Slade results in a Slade boulder and a Gabbro staircase).
As far as I know, there is no way to remove a dug Up staircase unless there is a place to stand beside it, so you can't remove a dug staircase to replace it with a built one to repeat the digging downwards. However, with the SMR you should be able to first use the trick to smash a magma flow down one level and then apply the built staircase in the hole followed by a dug one to make a two tile deep hole without any miner sacrifice (but that also destroys the magma flow, so you can't continue downwards using the magma flow method).
The above is basically consistent with the method of digging through the SMR (and Slade) from below using the digging of a staircase, with the difference that that method never fails, as far as I know.
Does anyone know what causes some digging to result in a staircase and some to just reveal the tile?