see when air reaches its dew point then we have to switch on the wet adiabatic dry rate.
up to 2000m, the air is above dew point so we subtract dry adiabatic rate(10) however when it reaches dew point we start to subtract the wet rate(6).
answer1: 2000meter is height of condenstation lifting.
anser 2: its conditional instability since below 3000m the air is stable but above it , it is unstable.
answer3: yes, its forceful uplift in effect of cold front underiding the warm air so thunderstome may be possible.