First, let's try the easy approach. Add a Solid Color layer of pants colour, set to "Color" blend mode, masked on to the pants, and see if it works.
Then try the same for the shirt.
Let me know if it's not satisfactory.
People get all hung up on enlargement method. The enlargement method doesn't matter. What matters is the sharpening afterwards.
https://www.damiensymonds.net/trainingsharp.html
The clone stamp is the correct method, yes.
But it sounds like you're not doing the masking with it? The masking is crucial.
https://www.damiensymonds.net/2010/10/role-of-masks-when-cloning.html
Lightroom has an inaccurate histogram and clipping warnings.
The histogram can be fixed with soft-proofing, but the clipping warnings are STILL wrong.
Once you actually understand colour spaces and histograms, you'll understand that Lightroom is completely unusable for this reason.
Anybody you meet who says that Lightroom is ok DOESN'T KNOW HOW TO EDIT.
Now, talk to me. Do you still think the body needs to be darkened?
Or do you share my point of view that the sweet spot is actually somewhere in between the two areas? I think the body should be darkened a bit, and the face lightened a bit.
Ok, can you update your profile details with your calibrator information?
You definitely didn't need to do all that stuff with turning the blue areas to grey in the background. You're simply going to add a black Solid Color layer and carefully mask it around the whole horse.
And you definitely need to do that first, before we discuss the imbalances in the coat.