The healing brush tools, I've found, are good for small areas and spot repair where the tool has a larger surrounding area to pull from, ie small blemishes on a face.
For larger repairs, I stay with the clone tool. Keep the hardness down and also lower the flow to blend into areas. Possibly what's happening is you're working with too hard of a brush/clone tool and that's why you're getting hard edges around the areas you've repaired.
I've also found, especially when working around edges, use the pen tool or one of the selection tools, to select an area to repair. This creates a "border" so when you're filling in, it doesn't go beyond
the edges of the selection. Gives you a bit more freedom.
This isn't a tough image to repair. It does take some time, patience, attention, and flexibility using the tools.. And step away from time to time - your eyes get tired and it's difficult to make an accurate repair.
Often, when working on images you're attempting to restore, put some work in, shut down, and then look at it again the next morning. You'll end up seeing things much differently.
Just some thoughts from an eternal student...
- Jeff