OK, long story short:
Suspensions need to move. Specifically, arms need to pivot, and because pivoting arms move in arcs, anything that is attached to an arm needs some way to accommodate the changes in angularity that come with the arms moving along arcs.
Any metal-on-metal pivot will need lubrication, or it will bind and or squeak.
Binding and squeaking is bad. Customers do not like binding and squeaking.
But anything that requires regular maintainance is also bad, as your average car owner is pretty crappy when it comes to regularly servicing their car. So suspension pivots need to be self-servicing.
If you make them out of solid blocks of rubber (such that all the pivot motion happens by rubber deforming in torsion) you eliminate any sliding motion. No wear, no opportunity for squeaking. And by tuning the hardness of the rubber, you can damp out certain high-frequency vibrations and really cut down on noise transmission.
So the solid rubber suspension bushing is nearly perfect from an OEM point of view.
Answering your questions then:
1) The bushing's purpose is to flex (usually) in torsion, thus allowing the suspension to move.
2) This question makes no sense: a healthy bushing sees no slip; the metal embedded within it remains fixed to the rubber, and the rubber deforms to accommodate movement.
3) Time, UV light, and ozone - but mostly time.
4) A bushing is a pivot point. A cover is a cover. A stopper is something squishy designed to limit movement along a linear track.
5) The terminology is not consistent manufacturer to manufacturer.
See
http://farnorthracing.com/autocross_secrets.html
DG