I did it because there was a dog in front of the subject's arm in one of the photos
I separated the subject from the background in their own layers
I filled where the dog was
I took an arm from another photo in the same shoot and put it behind the subject so it continued. it lined up properly but there was some colouring issues so I wanted to match it with the layer. In hindsight i likely didn't need to do that, however, the youtube video I watch to learn from told me to do it.
Anyway, I merged the layers and nothing changes. Would it be better to just revert it to two layers? I'd likely need to do that to figure out pertinent Panels before and after, correct?
Please post sreenshot you are asked so we can see what is or what can be problem. Like this we can only guess why you can't do something that normally we can. If you have merged layer what is problem? What you can not edit? Usually you can edit any layer beside locked and smart object layers which will not allow you for direct edits.
From what I understand you want to edit colors, do you use Hue / saturation non-destructive adjustment layer?