Sanderson supports the laws of conservation. So even though stormlight is readily available, the energy required is so incredibly high that perhaps even a Shard wouldn’t be able to power it. But the reason amethyst wouldn’t work is bc of imbalanced forces. Even if they are bound to each other, the parent object can and will rotate when touched. Even if this is minuscule rotation, it’s enough to set off the “ow that gemstone just exploded” rotation. So I doesn’t necessarily cancel out, like you say, but instead gets rid of the source of both forces. (Another note. The forces aren’t divided between the two. Otherwise a spanreed would feel only half as much gravity.)
One interesting thing you bring up, though, is rubies. The forces don’t logically cancel out, but is it much of a stretch to say that at some point in time they have had the same force apply to them? No, absolutely not. But something that is mentioned over and over with spanreed me is frame of reference. Since the entire system is affected by gravity, then the frame of reference adopts gravity, and the spren no longer thinks it’s other half should be affected by the gravity it is feeling. (If any of this makes sense. I get the feeling I’m rambling. Also, soorry if I’m being a joy killer. I do think these ideas are cool.)