@sacredhart I can't answer all of your questions but generally I'd avoid turn-by-turn navigating with Garmin. once you miss a turn it starts recalculating and that takes a lot of time. best to just load a gpx, choose your map display settings and follow the line of the track. you cannot do this with a 510, that one has no map display
[edit: woops, hadn't looked at thread for a couple of days, answer obsolete]
You can disable recalculations or make it prompt to recalculate. Turn by turn means you're less likely to miss turns in the first place, that's the point. It also means you don't have to stare at a line on the unit.
@sacredhart I can't answer all of your questions but generally I'd avoid turn-by-turn navigating with Garmin. once you miss a turn it starts recalculating and that takes a lot of time. best to just load a gpx, choose your map display settings and follow the line of the track. you cannot do this with a 510, that one has no map display
[edit: woops, hadn't looked at thread for a couple of days, answer obsolete]