Connecting a Shelly 1 to a 3 - 4 - 3 way circuit

I purchased two Shelly 1 devices a few days ago and successfully set one up on a single-pole switch. I've been trying to get the other one connected to a 3 - 4 - 3 way circuit but I haven't had any luck yet.

The switches are as follows:

Switch_A:

Common: RED (assuming this is the LOAD for the lights)

Travelers BLACK and WHITE wires.

Just those three wires are present.

Switch_B:

4 screws, left screws have black wires, right screws have white wires.

There are two bundles here, one with 3 wires and one with 2 wires. The RED wire is not used in the 3 wire bundle.

Switch_C:

Bundle_1: 3 wires:

RED - to Switch_C Common terminal - seems to be the HOT wire

BLACK - wired to Bundle_2 BLACK

WHITE - wired to Bundle_2 WHITE

Bundle_2: 2 wires

BLACK - wired to Bundle_1 BLACK

WHITE - wired to Bundle_1 WHITE

Bundle 3: 2 wires

BLACK - to Switch_C Traveler terminal on Common side

WHITE - to Switch_C Traveler terminal on opposite side

When I disconnected the two pairs of wires in Switch_C's junction box, I could still turn the lights on if the other switches were flipped a certain way.

[L] - RED wire from Switch_C - Jumper to NEW HOT wire nut

[I] - to HOT wire nut

[I] - to HOT wire nut

[N] - to existing WHITE NEUTRAL wire nut

I have been trying different configurations. I think perhaps the problem is with the [O] terminal on the Shelly 1. It would seem I have to connect both travelers from Switch_C to get it to work. I guess this is a problem? I haven't tried it but I assume so.

Could this circuit work with a Shelly 2 somehow? Or is there a way to get it to work with a Shelly 1?

Thanks for any help!

Okay, I did my best! Attached is the information I have at the moment as a PNG file.

3 Switches:

Green 3-way <=> Blue 4-way <=> Red 3-way

Green 3-way -- to Lights / LOAD -- Red common wire, black and white traveler wires

Blue 4-way -- wired between the other two. Two black wires on one side, two whites on the other

Red 3-way -- to LINE -- Red common wire, black and white traveler wires <-- Target location for Shelly 1 (or whatever Shelly might work! I'll find a use for the Shelly 1!).

I would like the Shelly to be wired in in addition to the existing 3-way switch. The switch is a USA simple toggle switch, not a momentary or Decora / newer wide switch -- I mean the simple ones that have been around since before the 70s at least. I could change the switch type but I'd rather not.

There are 3 wire bundles in the target switch (the RED one) -- a 3 wire and two 2-wire bundles (all have ground wires and those are tied together). I am not positive where they are going but I have a general idea at this point.

Note: The junction box mentioned in the diagram may or may not be involved. I can see it but I cannot access it -- it is behind a heating duct very close to the ceiling and wall. It may not affect the wiring regardless? I am unsure.

-- Red common wire, black and white traveler wires.

EDIT: When I disconnected the cables in the RED box (the two blacks and the two white wires), the switch still partially functioned. But maybe you guys can see that already...

Thank you very much!