dougettinger wrote:The precession of Mercury's orbit helps to prove Einstein's GR. Do the precessions of the other planetary orbits also contribute to proving GR ? If not, then why is this the case ?

The perihelion precession of other planets due to GR has been calculated, but I don't know if it has been measured accurately. Mercury is a special case in that it orbits so fast we can fairly easily see a small shift (and the GR component is very small compared to other effects which create a much larger precession).

Trying to detect a GR-induced perihelion precession rate on the order of an arcsecond per century, embedded in a precession rate at least ten times higher from gravitational perturbations, is very difficult when orbital periods are measured in years.