Thank for sharing your concerns. We recently added a DataToolTipLayer which exposes a GroupingMode property join the values where the mouse position is located for each series under one tooltip. This is something I wished was part of our CrosshairLayer or if we create a new DataCrosshairLayer with this option. I strongly recommend using the link above to submit a feature request as there is currently no known workarounds to circumvent this. 

SplineSeries only wor with X CategoryAxes thus i could also use line series with interpolation.

This is not an option for my program because it has to display scatter time series.

Switching off the multiple annotations is switching off the behaviour which I want. This is paradox.

Again, to snap at identical X-values you do not need interpolation.

This is to be expected with a ScatterSeries. The crosshair is designed to flow with the mouse position hovering over the series, not the plot area. If the ScatterSeries supported UseInterpolation (which is currently does not) then you wouldn't see the snapping and crosshair would appear relative to the mouse position. Because there is snapping, having more than 1 crosshair visible is not recommended.

What I would do personally is change the hook the chart's SeriesMouseEnter/Leave events and change what the TargetSeries is assigned to so that only 1 crosshair is visible at a time depending on which series is hovered. If you want the crosshair to move more freely on the plot area (not on the series directly) then I would use a SplineSeries and not a scatter series.