How is the fast part of x12/x13 decided

[D10]

I wonder if a passenger can request that a driver uses a certain route (ignoring scenarios when they've been told to use a certain route because of traffic, etc.)?

As it wouldn't affect a passengers ability to board or alight, I would hope not. Leave the driver to make the best decision without being bothered by passengers.

[Rachvince53]

I wonder if a passenger can request that a driver uses a certain route (ignoring scenarios when they've been told to use a certain route because of traffic, etc.)?

They can but the driver is responsible for time keeping and ensuring the majority of passengers get to their destination ASAP. For the most part as long as the driver explains why they're going a particular way most passengers would surely accept that. I know some won't be happy but to put it crudely, at the end of the day the driver is the one with the steering wheel.

[2206]

For the most part as long as the driver explains why they're going a particular way most passengers would surely accept that. 

No reason for the driver to need to explain why they are following the NLR.