Neither approach. Children train when they are ready not because you tell them it is time. The worst thing you can do is try to force them at any time! If they train when ready, there will be no serious issues whether before or after the newborn comes.
It depends. Train before, if there are several months between the completion of toilet training and the arrival of baby. If time will be limited, I would advise waiting until after the new baby arrives and routines are developed. Otherwise, you run the risk of toilet training relapse or refusal. It is normal to see some regression of skills, with the arrival of baby-be patient with your first child.
Depends on maturity. Toilet training is a process that requires baby realize when they need to go, which differs from knowing they have gone. If they show interest, assemble the materials & walk them thru the sequence of steps needed to go, then tell them you will help. Give them the big boy/girl reputation with success & they may not regress when baby comes. Some envy the time you spend diapering baby & regress anyway.