They way I do it is left drag a clip to a new location while depressing CTRL. This creates a duplicate of the clip at the new location. If you want to restrain the horizontal or vertical position, depress SHIFT as well.
Depends what data is actually present that is causing the dead air at the end of the song. Look for MIDI events. Look for automation events. There may also be markers or even time signature changes. Tempo changes are also events to look out for.
If you have many tracks to check, you can select all tracks. Work out where you want the track to end. Put the Now marker to that point. Then go Edit | Select | From=NOW. Then hit delete Special. Make sure you select all boxes there.
Also under the track menu, make sure Options | Stop at project end is selected.
Promidi's post in Can You Duplicate Clips In Cakewalk was marked as the answer