I am replacing the switches for my basement. I have one switch (switch A) at the bottom of my stairs and another at the top (switch B).
Between the two switches is 14/3 Romex with red attached to common. Switch A also has 14/2 coming in from the light/panel. I have 2-1 Zwave 800 series and aux switch, so switch A will be smart and switch B will be aux.
So if you have the dumb switch wiring in the grey graphic you posted, that is just a variation on how a 3-way can be wired. More complicated, IMO.
The Inovelli wiring diagram does not show the dumb switch wiring in that configuration, so you have to adapt a bit. This is a non-neutral installation.
Box A - Inovelli
This has the 2-wire coming from the light and a 3-wire going to Box B.
The Load is the black conductor wire nutted with 3 conductors. Remove the wire nut and attach the black from the 2-wire to the Load terminal on the Inovelli. Attach the black from the 3-wire to one of the Inovelli’s Line terminals. Pitch the pigtail.
The red is the traveler conductor. Attach it to the Traveler terminal on the Inovelli.
Remove the wire nut from the white bundle. This is the incoming hot. Attach the white from the 2-wire to the other Line terminal on the Inovelli. Cap off the white on the 3-wire. It isn’t used. Pitch the pigtail.
Box B - Aux Switch
Red to Traveler terminal. Black (hot) to the Neutral terminal. Cap off the white.