Yes these are possible tweaks to show a cautionary key signature at the end of a flow even in a condensed score. But the procedures hide any time signature regardless of any property settings like Ends Voice and Force Position and Duration. Also hiding the rests in one of the suggestions is not possible in a condensed score not even by scaling them.
As far I’m aware of the only way to show a cautionary time signature at the end of a flow in a condensed score is a cutaway start (in Dorico 6).
But there is a flaw: Even with the minimum width (a quaver pickup) there is a gap between the cutaway start and the right margin and it’s impossible to shorten it in Engrave Mode.
I guess it takes a serious effort to implement real cautionary time and key signtures at the end of a flow but I’d be satisfied if the cautionary time signature doesn’t get hidden with the system track trick.
The last measure of the flow has the time signature 3/4,1. The right barline of this measure is hidden by setting Notation Options > Barlines > Automatic barline at end of flow to No barline. Each flute has a quarter note in this measure with its stem and notehead hidden in engrave mode. Finally, the width of this measure is reduced in note spacing mode.