I'm putting my money on the cleaning and re-lube doing the trick. "Nearly LNIB" could also mean bought, shot a few times and then put away and ignored (!!) The fact that you found a bit of dried lube inside is a good clue. Removing the stocks and soaking overnight in mineral spirits, then flushing out, drying and a light oiling can sometimes do wonders.
Good job on making your own rebound spring tool. I did the same, using a piece of steel rod, putting a "crank" in it and cutting an appropriate slot in the end. Stuck into a "Skrooz" file handle and works like a charm.
I got a 19-4 6" a couple of years ago and although it worked mechanically just fine, it felt kind of rough. I took the side plate off and was surprised to find that the rebound slide was slightly pitted, as was the frame behind it. No actual rust, though. I set to work with w&d sandpaper & oil (a little 320, then 400 & 600) and got as much of the pitting off as I felt was prudent, then did a littie other polishing with 1200 w&d and/or green chrom. dioxide on a felt bob, flushed everything out, re-lubed and reassembled and it was quite a bit better. Later I even put trigger & hammer shims in. Still not as nice as my 14-3 but pretty good.