Is this a reasonable, robust way for detecting voltage crossing a threshold?
Is it a reasonable design?


No, it looks too complicate to me. A TL431 (and a 2-resistors voltage divider) can replace the left half of your diagram, using 3 components only.

The right part can be mostly replaced using gates made out of transistors or diodes. A NOR-gate, as an example, can be done with 1 transistor only. Or you can use a normal transistor and a depletion mode one, both bases activated by the TL431 (ie: when voltage is high, the TL431 turns on the normal transistor, and turns off the depletion mode one). Beware TL431 can't supply much current, but you can use mosfets... 

Just a quick circuit diagram to give you the whole idea:



In this example you don't need the DC-DC converter to get the 12v and if you add a little transformer you can isolate the low voltage part from the high voltage one (safety). The optocoupler is there not because there's the need of it, but its transistor can drive bigger loads than the TL431 alone; I placed the optocoupler instead of a transistor just to introduce the idea of using a transformer to isolate the high voltage part from the low voltage one.

Resistors are in circuit, so they will burn some current, but considering the load made of 2 leds and a transistor, isn't much. Just be sure their power rating is enough.
In the case you need more than 1/4 or 1/2 watt resistors and you can't source bigger ones, you can split the power rail and use one resistor for each of the devices in the right part of the circuit. Just be sure that those devices can work with high voltage, or you'll have to make more voltage dividers. All this, of course, is to avoid the DC-DC converter as you asked in the comments.