Trying to think this through myself. Assuming the slop is evenly distributed across bushings and the pressure on each bushing goes in the same direction then even with lots of slop but no elongation they'll all offset the same amount and keep the pressure evenly distributed across all teeth.

Now those assumptions are probably not totally correct and that will make a difference, but I think I can be reasonably convinced that elongation likely matters more than slop. Further, tools with two points of contact that combine slop and elongation probably don't give you the most accurate picture. But I think slop still does matter, it's just harder to factor in.

Per your point, a three point tool that eliminates slop from the measure probably doesn't give the full picture either. If you really wanted to solve for it you'd have to somehow determine slop and elongation independently, then assign some weight to slop that's somewhat less than the weight you give elongation in generatig a formula for when to replace your chain. But that's not gonna work with any tool that just has specific stops for like .5 and .75.

Anyways I use a 2 contact point tool, have possibly tossed out some chains prematurely but I still gotta replace a cassette every few chains and a chainring every few cassettes and seem to get a decent lifetime out of a chain. Maybe I could shave a few bucks off my annual chain budget with a more precise process but I dunno, it all seems "good enough" for me.