3) I don't know if the tables in the motor specs allready take into account the inefficiency and friction of the gearhead yms.. If they don't, that is another force eating your power. Gearhead can eat easily half the power in small motors.
I would go for bigger motors. Something like this:
https://www.robot-electronics.co.uk/htm/emg30.htm
The motor you are considering is quite cheap, so I think there is not much risk trying it out. The motor I suggested cost 4 times more, because it comes with sensors attached.
If you wan't to find a cheaper, but bigger motor. Look at the specs of the motor I suggested for advice. I have used those motors in a 2 Kg robot which had 10cm wheels. The motors performed well, but they didn't have any extra power.