Firstly, about the lag issue, well, cave-ins don't necessarily happen right when you dig a cave, they might happen weeks or months later. So the code that tests for stresses and buckling might only have to execute 3 or 4 times a year, randomly. You could also factor in a heat-change that would permanently 'shock' a system, and would act as a trigger for localized checks. Make it so it doesn't have to redo a check in an unchanged sector and you're golden.
The delayed situation would also allow an interesting choice.
* Chrispy says "There was a battle in shaft 19, and the supports were shattered! She's mighty unstable right now, so I need a few brave dwarves to follow along so we can bolster up those walls for the whole section goes down!"
Or you know, just wait and let it collapse itself.
* Chrispy screams as the magma burns...
It would make sense to me that for any kind of mega construction, if you started to work with blocks instead of stone, you'd probably be able to make something that was a bit less... foundationally orientated. Bars of course would be stronger still. Maybe with just constructed rock, it would be impossible to build downwards or out to the side?
Also, what kind of cave-in system would be able to accommodate constructions that looked like a 3 or 4 legged stool? Or maybe a stalactite? What would it do in the case of an inverted cone that was resting on it's tip? Would that bottom block blow out, or would it just be the worlds craziest hairpin-balanced thingamabob?
As an argument for a 'different' kind of cave in system, would be something made around fault lines, or rock slabs. I imagine these to be lines in the rock which provide no adjacent support. So if you build a structure off the side of a steep hill that is itself not adhered very well to the rock below, it would give way in an avalanche. That would play hell with adventure mode tho, as you'd need to create the world 'post-collapsed/cavein'ed' for every new area, not just at embark.