I believe Apple Photos does not allow images to be just imported as reference without duplicating them in the library but Apple Support would be able to give a better idea for this.
If you have Creative cloud subscription, then you can use Lightroom Mobile to sync the images from your Lightroom Desktop to Lightroom Mobile on iPhone so that they are not duplicated.
Would this still work with 'Archived' Photos I have in lightroom but store on an external HD?
If you have those archived photos in Lightroom Library, then yes you can create collections for those images and sync them across to Lightroom Mobile.
Refer this article to get started with Lightroom Mobile : How to get started with Lightroom on mobile | Adobe Photoshop Lightroom CC tutorials