The trail map is simply a plot of your route that can be shown as a metric in a cell. It is useful to see an overview of where you have been but is pretty small and so not great for following your route back.
For this it is better to use the proper map, which shows breadcrumbs of the whole trail, making it easy to follow back. Even if you are displaying a screen format without a map then you can always double tap the screen to show the full screen map (unless you have configured double tap to do a different operation).
The off course warning currently only works for GPX routes. So if you load a GPX route into the app then the app will warn you if you deviate from the route (and when you return to it). No-one has asked for an off-route warning where the current workout trail is the route, but I can see how it would be useful. If more people are interested then I may add it.
As @rbart says it is possible at the moment, although you would need to end the workout when you get to half way; then create a GPX route from that workout; and then use that route during a new workout for the trip back.
