To leave a gap just drag one of the clips along the timeline. If it snaps back, try holding down <alt> or <shift> while dragging.
To insert another clip from the bin, first use the
keys to move the cursor to a split point.
Then right-click on the clip to be inserted and Place On Sequence at Cursor... 
Or do the same by using the ADD button... 
Export will always contain a video track. But the sequence audio can be saved by clicking the Sequences tab above the bin.
Then right-click on sequence 1 and Save Sequence Audio as New File... 
If will be saved in the MUSIC folder on the PC by default.
To remove a split, right-click on the split bar...
The drop-down menu may have an UN-SPLIT Clips choice. If not, that option isn't available.
Click the ? in the upper-right corner for the HELP files.