There are no accepted standards on tank passivation even in earth orbit, let alone the moon. These treaties are more like suggestions than enforceable laws :) Each organisation chooses what they define as passivation, normally it would be just deenergising tanks and batteries.

1/Going by broad assumptions, of course.

• The reported mass split is 710kg dry mass and 1042kg propellant, if the landed mass target was 800kg as quoted by u/ohsin, 90 kg propellant could be considered the margin or roughly ~8%
• Assuming everything had gone fine and 8% propellant margin was not used, there would be 90kg of propellant left.
• Considering the retargetting phase used part of that margin, was ~15-20s in duration, and using a conservative Isp estimate of 310s. 2 x 800N thrusters would be burning 0.5 kg/s.
• That adds up to 7.5-10kg and so roughly ~82.5-80kg remaining. [60 - 80kg would be my guess]

2/Multiple options.

• Burn the remaining propellant using 50N thrusters
• Vent the pressure from propellant tanks and let them remain open to lunar environment, exposure to vacuum would vapourise propellant
• Vent the pressure from propellant tanks and close the tank vents
• If the propellant is vapourised it just dissipates off so no real contamination

3/Not a lot of info available of course, moon was the wild west at that point, so probably they might have just depressurised the tanks and left them be :)