1). The bearing retaining system is known as a cam-lock collar system. The collar is off set, something similar to a cam shaft lobe. When it is rotated, it will bind of lock to the shaft. The set-screw then only needs minimal retention properties because the force of the cam action against the shaft rotation does most of the work. This is why you need to rotate the collar to remove.
2). Short answer is that the wear in the pin/yoke will decrease the belt tension. To what level depends on the wear. However, keep in mind that the lever spring-loaded over-centering action can compensate for some of the wear.
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