If you can assume the 555 will work as a latch (it stands to reason it should - it's just timing an infinitely long delay), how would you clean up the bit in the middle of the original circuit?

If Thresh goes > 2/3 VCC and trig > 1/3 VCC, the 555 output will latch Low.. if trig goes < 1/3 VCC, and Thresh < 2/3 VCC, the 555 output will latch High. /Reset over-rides all other states.

It is possible to use the 555 as a S/R just using thresh and trig inputs.. pull trig high with a resistor, pull thresh low with a resistor, capacitively couple +ve pulse (or rising edge) to thresh, capacitively couple -ve pulse (or falling edge) to trig..

A -ve pulse (edge) on trig will latch the 555 output high.
A +ve pulse (edge) on thresh will latch the 555 output low.

 

A -ve pulse (edge) on trig will latch the 555 output high.
A +ve pulse (edge) on thresh will latch the 555 output low.

So you're saying that with a few changes I could have done without the transistor?

Anyway.. I put it together as shown in the last diagram and it seems to work:

The tie posts are for the reset switch and a panel led, plus an unrelated change.