>1. Would it be feasible to simulate muscles by twisting chords using electric motors to shorten them, or simply reeling up cable/chord?
Sounds doable. I've been trying my hand on a similar design.
>2. If so, would pairs of these "muscles" working opposite each other, like biceps and triceps, be able to regenerate electricity as one pulled against the other to unwind/unreel against the opposing motor? Obviously there would still be energy loss but could you reduce the loss by using motors as regenerators?
Wouldn't work. Any energy the relaxed engine would generate would be extra energy the engine under current would consume. The reason stuff like regenerative breaking works for EVs is because you're taking energy from the wheels while you don't want the wheels to spin.