Hi gents,
I think the light areas might be where the concentrated gel stripper was first applied and stayed on a bit longer I would suggest giving it one more stripper coating and then taking another look.
IF you have to use 400 grit paper, cut it into inch-wide strips 8" or so long and wrap it around a file to guide your sanding. Do not just use hand pressure as this will give it too much waabi (in other words make the surface wobbly). Do NOT sand the entire stock - just sand spot areas where you find more problems than you are willing to live with.
Personally, if it were my stock, I would re-strip and then finish it - leaving any remaining discoloration as pat of the rifle's history.
Cheers.