How can an accretion disk form around the central star of a planetary nebula?
The surrounding gas has already escaped, mostly during the thermal pulsation phase of the AGB progenitor. A common envelope phase just accelerates that process.

The envelope is relatively easy to remove (at only a few km/s as you say) because it escapes from a position a long way (several au) from the centre of the star where gravity is weak and is driven by radiation pressure acting on dust that can form in these cool outer regions.

In contrast, the gas stripped by tidal disruption of the planet is close to the core of the star, which is only a few thousand km in radius. The escape velocity there will be thousands of km/s and the gas will be fully ionised and comparatively transparent to radiation and hence radiative acceleration.