How can I force pages to update images to reflect changes made in external editor?
Is there a simple way to force this to occur, or am I forced to re-import the images and hence have to re-size and re-position them accordingly?
This won't happen automatically in Pages. You can save some of the work by setting your current versions of embedded images as Media Placeholders. Then when you drag new copies into those places the sizes and perhaps other features will be retained.
Format > Advanced > Define as Media Placeholder.