P: Apply edits made in Lightroom to device library

Adding another benefit this would bring. I spent some time reprocessing some old photos in HDR. Then when you export the updates they of course get saved as new photos and thrown to the top of the Recents Album, which is what many apps use as the view for the photo picker. 

I know this is 100% an Apple issue, but is another case where this ability would be a nice option to have. It would also save me from having to go back and delete the previous exports.

Another benefit of this would be more clarity when you are on the device tab. Anything you exported from Lightroom and added to Photos isn't marked as imported into Lightroom. While the reason it isn't is obvious, it makes for a confusing workflow at times. I've accidentally imported photos that were actually exports of an edited photo a handful of times. Creating odd duplicates and confusion in my LR library. 

Got it! I really like the idea. It's probably a jpg that gets saved/layered on top there. Given that Apple now supports Adobe's implementation of JPG+gain map HDR, even better. Just adding "save to original in Photos" as an option in the "save to device" sheet would be great, for any photo LrM IDs as linked to an image in Photos.

Exactly. iOS is specifically built to allow this. There are apps like Photomator and Darkroom that allow you to view and edit your Apple Photos library. Extensions can be used to do this right from the Photos app, but I think they have kind of fallen out of favor.  When you finish the edits they do just what you said. In the background export a new new version and save it on top of the original. All this is done transparent to the user. 
If you later decide you want to revert the change you simply click the Revert to Original button in Apple Photos. Most of these outside editors allow you to revert straight from them as well. 
since Lightroom already seems to be tracking what photos from the device library you have made edits to inside of Lightroom it would be a nice extension of that feature. 

From outside app non-destructive edits, do you mean the "Extensions" portion of the editing pane in Photos (e.g. where PS Express shows up), or are there other instances that you're thinking of that use non-Apple markup/editing tools and actually save the edit to the image itself? I feel like I have done this myself but can't remember what apps do it.

I agree that it'd be nice to be able to more easily manage a Lightroom-edited version "alongside" an original Photos image! I'm not entirely sure how that'd be handled, though. I think there is a method by which a jpg gets created that sits on top of the original HEIC/whatever—that may be what you're referring to? Is it then that "reverting" changes in Photos simply ditches the jpg and goes back to the original HEIC?