Your plan seems to lack... planning.
For (semi) creature-proofing, you dig to 2 short of the magma. Smooth that 2nd to last, then carve it into a fortification. Then, from above, channel out that last bit.
If you mine straight in, say b'bye to the miner. Only a superdwarvenly agile miner can outrun that, and then only rarely.
You have to find a spot where the magma you want to tap extends out beyond the layer above it, to channel
down to tap that - only safe way.
You can channel down into hot rock - the magma won't well up (possibly unless it's been pumped to where it is, and that's a more complex discussion.) So if you're talking
channeling (<d>, <h>), that's safe. If <d>, <d>, that's a death warrant. But your magma will flow.
If you have an entire "plan" counting on this, then 1) you learned an important lesson, and 2) you're about to learn another - either how to sacrifice dwarfs, or how to use pumps to move magma.