No, you're actually fine on that front here
The C-D conduit meets 300.3(B) despite not having a neutral in it for the same reason old-style switch loops were compliant with that rule: all the power is coming back via the same wiring path it went to the switch on. In other words, current in on the hot = current out on one of the two travelers, which are both in the same conduit as the hot wire, thus there is no issue with a stray loop.
However, there's no need for the gymnastics with the neutral to the add-on switch
However, your plan does something rather needless by running neutral all the way from the main switch's neutral terminal to the add-on switch. Instead, you can simply tap the neutral connection for the add-on switch off at the light fixture box instead by nutting the white wire from C in with the rest of the neutrals there.
This also is a chance to fix the Code issue that does exist with the current wiring
There is one problem with the existing wiring though, and it has nothing to do with improper wiring loops, but with color. You see, the reason you're permitted to remark a white wire in a cable as a hot is because you can't get cables with arbitrary-colored wires in them without going to great length and expense to have them custom-made for you. However, that's not an issue in conduit jobs, where the electrician gets to choose which wires and colors to run when they pull the wire through.
As a result, that white traveler wire violates NEC 200.7(C) point 1 in two ways:
(C) Circuits of 50 Volts or More. The use of insulation that is
white or gray or that has three continuous white or gray stripes
for other than a grounded conductor for circuits of 50 volts or
more shall be permitted only as in (1) and (2).
(1) If part of a cable assembly that has the insulation permanently reidentified to indicate its use as an ungrounded
conductor by marking tape, painting, or other effective
means at its termination and at each location where the
conductor is visible and accessible. Identification shall
encircle the insulation and shall be a color other than
white, gray, or green. If used for single-pole, 3-way or
4-way switch loops, the reidentified conductor with white
or gray insulation or three continuous white or gray
stripes shall be used only for the supply to the switch, but
not as a return conductor from the switch to the outlet.
Since you're turning the white wire in C-D into a neutral, and already have a neutral in the A-B conduit, you can simply remove the extra white wire from the conduit (slowly and carefully!), or abandon it in place by capping it off at both ends if you'd rather not fuss with it at this point.