Boy, I have to wonder about your "overly complex [and] prohibitively expensive" designs if this is the cheap and simple option!
"Two-stage" (not "twin-stage") is another term for
double action, but that's different. You appear to be using this mechanism to only register a single keypress.
Having a slider where the spring can be engaged and disengaged at will sounds possible, but it seems like it's an overly complicated replacement to
stacked spring. There are some fiendishly weird switches;
Alps SKFF series (vertical plate spring) for example.
If you look through patents, you'll see that there are so many of them for switches we've never come across yet (and may not have all gone into production), and all sorts of strange ideas. I don't recall seeing anything like you've described, though.
With so many things, it's not a case of outright "possible" or "impossible" so much as whether you can achieve the desired level of reliability within an acceptable manufacturing cost. I don't know how you imagine that the inner slider is going to disengage from its spring, considering the size constraints. I'm thinking that it might be more realistic for the inner slider to remain on its spring, and for it to hit a hard stop that causes the outer slider to ratchet past it, causing the inner slider to fly back up to the top. That would give you a click sound, too. The outer slider would need to be able to reliably re-engage the inner slider under very low force (close to the preload, so 30-45 cN).
However, that would give you massive "click hysteresis" where you'd not get another click until they key was fully released. So you'd need some other device to hold the inner slider down so that it can be re-engaged at a deeper point than fully home.
I like how the Romer-G leaves room for central illumination, but that of course comes at a cost of consuming much valuable space within the switch, so you have to consider whether your design would leave room for an LED (be that through-hole monochrome, through-hole RGB, SMD monochrome or SMD RGB). Also, what sort of switch contacts were you envisioning?