My decision to procrastinate with respect to the steam passages has worked, I think. Several suggestions have come in, and I am intending to go with the one from Stuart. And that is to angle the steam passages, which lengthens one on which I can use a larger diameter milling cutter, and to shorten the one under the steam port. See the red lines for the proposed changes.
While waiting for a light bulb to switch on regarding the dark places, I have not been idle.
I moved on to a part of the triple expansion steam engine build which I expect to be easier. And that is the condenser unit.
The condenser is the box shaped protuberance attached to the columns. I believe that its function is to convert the last dregs of steam, after driving the 3 pistons in succession, into water, for re-use in the boiler.
These are the components, machined and ready for assembly.
The holes in the end plates have 0.5mm of material between them. Tricky drilling, but a breeze for the CNC mill.