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You can use ola/uber to go around but if you are not able to find any then just hop into any taxi. If you end up going in a local taxi then only pay the amount which comes up in the cab meter, do not pay even a single extra penny.
During peak hours the traffic on Mumbai roads is crazy. I would suggest you to keep an hour and a half as buffer to have a relaxed visit to the mall.
Bus services in Mumbai shut by 12:00AM mostly so it will not be possible for you to get a bus at that time.