I'm just going to throw out a few of my own guesses
Had no luck tracking Ana'sDad illustrations. But when jfheath manages to revive some of his schematics, you'll picture it all in a flash, with no guessing involved.
I'm just gonna call the parts the cap, the casting, the steel, the brass, the piston, narrow spring, large spring, retainer.
So let's go with that!
.........seating the tapered surface [ that I'm going to call the valve ] against a machined surface [ that I'll call the seat] within the housing.
You like high-tech, but sorry, there are no metal to metal seals in the PCV. Sealing metal to metal requires high precision machining, high seating forces and "casting" is not the material of choice for it either. And you'd have to throw the PCV away once the sealing surfaces are worn. Only serviceable elastomeric seals in this valve.
So far, it sounds like you are going along with the concept of the three stages of activation?
1) Low and rising SMC pressure: PCV valve is open and pressure freely transmitted through it?
2) SMC pressure reaches Honda's set limit for the rear and the shut off valve closes? For illustration purpose only, let's say this limiting pressure is 300 psi.
3) SMC pressure keeps rising to a point where Honda has decided it is time to taper off some of the rear braking power? Let's go here with this higher pressure maybe being 600 psi.
Then you recognized the Brass spool, working against a small spring, and the Steel spool, working against a stronger spring.
Physics dictates that the spool with the weaker spring will shift first and then the spool with the stronger spring will come in second as the SMC pressure rises (neglecting some other factors).
So in the hypothetical illustrating pressures example, the Brass Valve would shift first against the small spring at 300 psi: Hence the Brass Valve is the Shut Off valve that initiates the second stage (constant limited pressure on rear).
And the Steel Valve would follow at 600 psi: This is the Decompression Piston that initiates the third stage (decreasing rear braking pressure).
If this seems to make sense, I can pursue later with describing how each valve works.
Below to illustrate the location of the components: