Yikes! Thats about the worst yellowing of needles Ive ever seen without the tree not being outright dead.
I have never seen yellow needles on any of my pines that were caused by "winter sun".
I have several mugos along with scots, JBP, JWP and pitch pines.
Its always been a fungus, scale damage, iron/nutrient deficiency, dehydration or root issues.
Its hard to tell exactly what it is honestly.
I think this is most likely a fungus, and looks like blight but Im not sure.
Closest thing I could find is:
https://www.canr.msu.edu/news/managing_dothistroma_and_brown_needle_blight_on_pines
The good news is that the new growth looks fine.
Not sure what you can do for those needles now, but I would start a regular application of antifungal every year as a preventative.
This tree is severely compromised and all those yellow and brown needles will probably die and fall off.
I would do no work or repotting of this tree until it recovers. At least 2 years I think.
Hope there is someone that can offer a better suggestion as to what this is
Good luck with it.