1)no
2)You really can't. Replating the entire piece is the only way to go.
3)It can though not always. The nickle plating is applied to a layer of copper as
nickle won't stick to bare steel. If the bonds break the nickle can peel. Some
times nickle peels just for fun..
4)Nickle will discolor if heated and I would bet solder won't stick..
Nickle is applied in 2 ways, eloectro plate and electroless plating. electro plating leaves uneven spots in the plate layer, electroless doesn't. Of course there are other finishes that are plated also such as hard chrome. They have differeing properties.