Please try not to edit on externals. That's fraught with risk. Externals are only for backup and long-term storage.
Were you cropping when these ones had their problems too? Or were you doing something else?
I'm not optimistic about that, sorry.
I still think cloning is the way to go. But yes,you'll have to be careful to vary your source point frequently, to avoid obvious repetition.
Hi, I've moved your post into @Brian's section, because he's the guru for camera-related stuff. He'll be along shortly.
I imagine it will help him if you can tell him all the specs of this shot - ISO, shutter speed and so on. In fact, it might be even better if you can show him a screenshot of those details from Lightroom.
Please don't say "4x6" and "8x12". They are sizes, not ratios. The ratio is 2:3, and that is what's important. Importantly bad, in this case.
No. You definitely must not make them smaller. Don't impose the 300ppi resolution. Leave the resolution blank, this is SO important.