IMHO that is a rough spot to get to, right in the corner.
Only thing I will comment on is I have used 000 and 0000 steel wool on many guns, as well as the old penny trick, I think you need a pre 1980 penny, whenever they quit using all copper. Nice and oily and not much pressure, can do wonders.
Story time......
Guy across the road left a shotgun out leaning against my shop. He came over and we are busting clays in my back yard.....I never saw it. He called me that next weekend, did I leave my shotgun over there.....no, I go to get the mower out and guess what leaning next to the door is his shotgun....oh god. I called him and said yea it is here and leaning against the door I found it when I went to get the mower....it looked like hell. Little rust everywhere and the wood looked horrid. It was his fathers and he was just heart broken. He was just going to stick it in the back of the safe and just cry over it.
Let me work on it for a bit, I think I can help it.
The wood got cleaned, ballastol, wax. The metal got the oil and penny trick as well as the 0000 (I think) it was not that bad, just looked bad and oil as well as a total tear down, and little parts tossed in the sonic cleaner.
Really I was shocked at how well it came out. My wife and his wife quilt and she picked it up, she said it never looked so good and he had been down all week. S happens, not to worry and anytime.
He was really happy with the results, but he is careful to put in his car what we are finished with that day.
There is a downside to this story however, you will endup getting more stuff to make work, and or clean up.