It's my understanding that temperatures below -24C result in moisture NOT sticking to the airframe. If this is genuine, then why does the Q400 detect ice at -33C? Is this a limitation within the sim or something?

Maybe it detects ice where it does not need to stick to the aircraft's skin, e.g. air inlets?

What I've noticed about this bird is that right before the detection warning appears, the flight dynamics change as if to represent the effect ice has on the control surfaces. This appears to be happening even when I'm in a 'no stick' situation (beyond -24C temps), so apparently it's not modeled to that level of detail (i.e. you'd get an ice detection warning when the air inlets vibrate, but there would be no change in flight dynamics without ice actually on the surfaces).

In real life flying, we were told ice will not stick colder than -40C. I heard that standard will change. In the flight manuals of the jets I've flown, anti ice will be on in visible moisture 10C to -40C or below 10C and the OAT and dew point spread is less than 3 degrees

Ice is actually simulated within FSX, so most ice detection on simulator aircraft is just reading a variable fetched from FSX. That would point to your weather engine being at fault, not the aircraft.

 

The definition for icing conditions I use is "Flight in to visible moisture (cloud, rain, drizzle, hail or snow) at temperatures between -20 and +10°c SAT (also mist or fog with a visibility of less than 1000 metres)". There is a very detailed explanation of the various types of ice and its effects at:
http://www.atraircraft.com/userfiles/files/coldweatheroperations_2011_20.pdf