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A suspension upright has six degrees of freedom: vertical translation, lateral translation, longitudinal translation, steering angle change, camber angle change, and caster angle change. The rear suspension must be constrained to a single unique path.
Six fixed length links would prevent all motion. Making only one adjustable (the spring) achieves one unique suspension motion. Making the toe arm adjustable (I.e. making it a tie rod connected to a steering rack) adds the ability to steer the wheel.
A 5-link is not over constrained and works fine with ball joints everywhere - that is what I am currently running.
I did not measure the stiffness of the steel arm itself because it was clear that at operating loads all meaningful deflection came from the bushings.
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