I think the biggest problem is that the amount of tension you need to apply to hold the binding pins can vary quite a bit, and it's hard to build a mechanical device that can feel with enough fidelity to figure it out.
Not really, once you have any materials issues with the vertical portion ironed out you just need a fine but rigid shaft within the body of the 'key' to bring it all the way out to where you have plenty of space to work. You'd need to have the shaft in a tight channel, but that's purely mechanical and should work just fine over even several inches.
not at all. sensitive strain gauges are commonplace.
I think if you actually tried to build this, you'd find that highly sensitive strain gauges are not sufficient. The human hand is extremely sensitive, and for example can detect tiny clicks and vibrations that you're not going to get with a simple strain gauge.