any benefit to doing a pair of 10R in series vs a pair of 39R in parallel?
Not that I'm seeing. Dissipated power in each is roughly the same.
Do you have the cable already installed? It's just because, you see, you are using 7 wires for a series connection. This can be done with 2 wires, by running the 2 wires from the board to the farthest LED passing by all the other LEDs, and at each LED break one of the wires and insert in it the LED.
I don't know if you're going to have trouble with cable length, a grey area to me. The frequency is relatively small, but the signal rise/fall times can be fast. You'll have to try, maybe scope it, and then if problems arise, maybe slow down the signal transitions speed by inserting a small capacitor from the base to the emitter of the transistor.