Sorry, my explanaition has been to sparse, I'm just a bit to deep in the stuff
A local coordinate system is represented by an GeoFeatureGroup. Everything that is a GeoFeatureGroup like a PartDesign::Body or a App::Part apply their placement to all their children. Everything that is within a GeoFeatureGroup lives relative to each other, just like FreeCAD is now, the objects share the same reference frame. However, once objects are in different GeoFeatureGroups they do not share the same reference frame anymore. But they don't know that. They can't anticipate the position of the other feature correctly. It is all about placements and how they correlate. This also means that for objects that are not Geometrical, everything that is not a GeoFeature, those problems don't arise.
1. Only direct links to GeoFeatures and GeoFeatureGroups are checked and handled
2. Expressions are not checked, they can still reference everything. This is a bit of a break of the principle when it comes to using placements of GeoFeatures from within an expression, but well, we need to accept that.
3. Spreadsheets are not GeoFeatures, they can be referenced from within a GeoFeatureGroup to everywhere
Regarding TechDraw I don't know how it is coded. I don't think it uses GeoFeatureGroups so it has no own coordinate system, but could of course be wrong. I think Wandererfan must think about how to handle Cross-Coordinate-System links for his workbench. It does for one make sense to allow a TechDraw sheet only for Parts in the same GeoFeatureGroup as the sheet, as then all Placement would be immediately clear (and moving the GeoFeatureGroup would not move the drawing). But this seems also a bit cumbersome and as an artifitial restriction for tech draw. Hence he could also port it to use the Cross-CS-Link properties and than make up his mind how to deal with the placement problems.