That's a good question. Bone growth follows muscle growth, function, and posture. Assuming a kid that is still in the growth phase applies pressure with his muscles as you suggest his bones will respond quickly and expand, maybe even change shape entirely. In the case of an adult however, his bones will respond slowly with remolding and will probably give him an ache as a signal to stop what he is doing :).

I don't know. There probably isn't . you see it in kids of immigrants. They are much taller and more robust than their parents. They supposedly have "short genes" but expose them to the highly caloric environments of the West and they magically sprout .

I have added 3 inches to my height since high school as well. I think we know very little about growth potential.

While there may not be strict limits to Wolff's law, there is a definite limit to the structure your anatomy is capable of maintaining. You can't go "beyond" ideal anatomical function, as you will simply revert to another kind of imbalance. In one video Mew was talking about how he had over-expanded one side of the patient and was now waiting for that side to regress back before continuing treatment. To me this implied that the reversal would have happened regardless of the efforts from the patient's tongue, as that side had expanded further than what the physiology of the patient was capable of maintaining.

Yea, I don't think it's possible to exceed the limit set by the muscles/soft tissue, but there are instances that defy this logic. One of them are people with deformed skull shape. I think since the bone is externally manipulated at growth and there are no muscles in most of the skull to reverse such manipulation it just sticks around.