Not really my cup of tea, but thought I’d have a dabble with it anyway. However one thing that would be supremely useful is some sort of indication in the Timetable top menu which routes support the feature, then again within the route sub screen which train types. Would save a lot of frustration and clicking then having to exit a run which only offers driver mode. In fact have the choice to play as Guard or Driver appear before the sim commits to loading the run.
That comes right at the end and if there is no Guard mode it just launches into drive. Hence my suggestion to flag the routes and trains prior to pushing the button!
You do not have to click on it. A service shows a ticket symbol on the right of its list entry, next to the medals, if guard mode is available on it. You can't do it on a per-route basis since it depends on the vehicles that the train is made up of. You also can't do it per-train since there are ECS runs, shunt moves etc.