In Garageband, is there a quick way of selecting everything to the right of the playhead, for instance to insert or remove a couple of bars in the middle of your song?

The best way to insert a few bars into the middle of a project (on all tracks simultaneously) would be to use the arrangement track and arrangement markers. You can mark sections of your song with arrangement markers, for example "Intro", "verse 1", "Finish" and then rearrange the sections by dragging the markers around. This way you can easily drag a few new bars as a new section between to other sections. This is described here:

https://help.apple.com/garageband/mac/10.3/#/gbnd8431c634