: English is not my native language, so perhaps you can help me with terminology:
: "pivot bushing" == "pivot cup" ?
This is correct.
:  I don't think I'll ever mount the plates differently than now. The concept of mounting short plates all the way to the front is said to offer greater maneuverability (smaller turning radius), but I think that should be the job of the steering mechanism, eg the angle of the kingpin etc.
: I also think that the argument of greater stability forgets backwards skating.
: When I land, I want to have the rear axle all the way at the rear side, and the front axle all the way at the front, but also in all other situations.
Actually the steering of the skate is done by the rear wheels (when moving in a forward direction). If the rear axles are mounted much behind the middle of the skater's heel (not the boot heel) your turning maneuverability is decreased. This may not be as much of a problem when ramp skating as one is following the lines of the ramp surface, not leaning right or left as when turning on a rink floor.
As far as stability when skating backwards, most of your weight should be over your front wheels while backwards skating. If you're placing much weight on your back wheels you will topple over at the first tiny pebble you encounter, no matter where the rear axles are placed. I actually skate backwards better now that I moved the plates forward. The example on rollerfr is a bit of an extreme example. Most skaters would probably not need to go to a smaller plate, unless they had very small feet. Simply moving the plate forward one half to three quarters of an inch would do the job. Virtually all North American made skates come with the plates mounted in artistic and roller dance style, which is useless for outdoor skating.