....Series wiring would seem to turn the entire rear light system into a single point of failure (and potentially introduce grounding issues) for little if any benefit. With parallel wiring, there's a level of redundancy.
Re redundancy, true, but it depends how the wiring is configured. I would be surprised if separate wires are run all the way from the headlight to each rear light in parallel operation on a trike; I think they would have to be commoned somewhere, eg at the headlight, which is perhaps the most likely point of failure.
Regarding ground faults IIRC the current topline model does not have a bolt that is a ground connection. Dunno about the secula but I think that also does not have a ground connection built into the bracketry.
Because of the low voltage required for red LEDs, if you were going to DIY an efficient setup, series operation for two rear lights would be the best (simple) way. For example I have a nice bright DIY rear light that only draws 30mA and I could have two in series quite easily.
IIRC at any given wheel speed if you reduce the current draw the voltage output goes up and using some headlights (with clever circuitry) this typically does increase the light output at modest triking (eg 12-16mph) speeds.
cheers