Ask the customer, "you want the shortest route, right?" and wait for yes. Now you have explicit permission to drive anything that's arguably the shortest route. So you pick one that passes the largest number of traffic lights and watch the meter tick as you wait in front of each traffic light.

Padding by 300% is a challenge, but a bit isn't.

They can and they do. The onus is on the passenger to report issues such as indirect routes.

Time for a "taxi driver" firmware for Garmin devices? They're pretty open iirc.