I want to shorten the length of the final bar in the line, without any of the other bars moving. Is this possible?
Turn on View: Handles. You'll see a marker just past the end of the last stave. Drag it across.
The other bars in that line will re-size to accommodate the shorter line length. If you want just one bar compressed, Shift-Alt-LeftArrow and Shift-Alt-RightArrow will compress or expand the spacing of just a selected bar or bars.