But in your written explanation there are a lot of things that Garmin users will find confusing.
So please excuse some comments.
A good concise description. Personally I am not a fan of using Google maps, but I know that a lot of people are, so this is very helpful.
Not sure what you mean here - Basecamp will not recalculate it. If it is transferred to the Zumo the Zumo should not recalculate it - but you have to make sure that there are non of the check boxes ticked in Edit -> Options -> Device transfer. Also that the map on Basecamp is the identical version to the map on the Zumo. Otherwise the route will recalculate when the Zumo receives it. With nothing else to keep it in place, it will likely result in a Faster time route.
I know what you mean - but being pedantic, tracks do not have any shaping points or Via Points or Waypoints. They have only track points.
(I know this sounds to be nit-picking but that isn't my intention. In Garmin Speak, those three types of points have very particular meanings, and they behave differently when navigating with the Zumo. Track points are just a dots on the map, joined together to appear as a curvy line.)
The route that you get when converting the track to a route doesn't have any shaping points either. On the video the original route on MyMaps has a load of Waypoints. Although the waypoints are transferred to Basecamp and they are shown on the map, the waypoints do not form a part of the route itself - so they cannot be made into Via Points which will alert you as you approach and arrive. Neither can they be used for the data display on the Zumo screen to give distance to Via or time to Via.
Instead, the route consists of what Garmin refer to as GPX Route Point Extensions. I call them ghost points. You cannot see them on the map, but you can see them if you load the gps file into a text editor. They are points which form the shape of the route between pairs of shaping / via points. Or in this case between the start and the end. They are the things that ensure that when a route is transferred to the Zumo, the Zumo receives exactly the same magenta line as was produced in Basecamp. And this will remain as long as the Zumo does not recalculate it. If the route is allowed to recalculate, all of these points are lost and replaced with ones which define a newly calculated section of the route. In this case the whole route will change from the original that you had in MyMaps.
If you add a Waypoint into a route, it can be defined as either a Via Point or a Shaping point. You can add such points without them having been Waypoints. In MyMap, you can add new destinations. They get passed to Basecamp as Waypoints. Any place where you simply click the map in MyMaps is not passed to Basecamp at all.
'Skip' is an option on the later Zumos that allows the next shaping point or via point to be removed from a route - so that you don't have to visit it. If it is 'Skipped' in this way, the stanav will never take you abck to it. It has gone from the Zumos list.
However, I suspect that you weren't referring to the skip function that is built into the XT. I suspect that you meant that if you add a route point in Basecamp and you do not visit that point when it is in the Zumo, it will then continue to try to take you back to it.
Yes it will. But Via Points are treated differently from Shaping Points. If you miss out a Shaping Point when riding, yes, the stanav will try to take you back to it, and it will calculate a new route to get you there. But if you ignore the instructions and join the magenta route AFTER the missed point, then the Zumo will continue navigating ahead.
Via Points are not as forgiving. The only way of getting the Zumo to not take you back to the missed Via Point is to press the 'Skip' button. Or better still, not have it there in the first place. Garmin often referes to Via Points as 'Destination'. They treat them as somewhere that you most definitely want to visit. They are not intended to be put in a location simply to pin the route onto a particular road. They can be used for that - but if you do, make sure that they are in a place that you will definitely be riding through.
As described above - you don't have to turn off recalculate for this to happen on the XT. But int he example you have given, there are no Via Point or shaping points to pin the route to particular roads. All of those flags that you put into MyMaps are no longer there. You have to turn off auto recalculate otherwise the moment that you miss out one of those ghost points - the entire route from your present position gets recalculated.