Identifying Wires (especially Neutral) in Complex 3-Way Light Circuit
I assume the orange wire coming out of S1B's common terminal splits at or near light 1, in the 2nd floor ceiling, providing the load for light 1 and also ultimately for light 2, on the 3rd floor ceiling, as the orange wire from D-E is unbroken. I couldn't find this particular configuration as an example of multi-light 3 way wiring, though.

Right. Most novices get intrigued by those &quot;color-by-numbers&quot; drawings of 3-way circuits all over the web, which are made for use with /3 cable.  Your installer took full advantage of being in conduit and having a variety of wire colors... by doing something totally bespoke that is correct for your installation.
In other words the original installer took a procedural approach to a 3-way circuit:

Connect both 3-way switches with 2 travelers (lavender here)
Connect always-hot (pink here) to the common of one 3-way switch.
Connect switched-hot to the other 3-way common and all lamps.
Connect neutral to all lamps.

Treat each wire individually and you can see what they did makes sense.
I believe you will find conduits C and D both go to the lamp box of lamp 1.  The (lavender) travelers pass right through the box.
I believe that lamp box also has the purple, peach and that other neutral passing through it.  These wires have nothing to do with your 3-way circuit; leave them alone. No, you can't poach neutral from that.  Neutrals must stay with their partner hots.
The good news is, this being conduit, you can have anything you want anywhere you want - you just have to pull it.  So you can pull neutral down from wherever C goes, onward to D if necessary.  However since you will then have 2 neutrals in the box, you must clearly mark which is which, e.g. by wrapping associated wires with a couple loops of tape.