I think option 2 is the best first step. You have to find out what the staining is before you can do anything with it. You certainly can't dye it without knowing whether it is mineral based or if it is vegetable based.

You don't say whether the staining is all over (in between the pleats) or just on the top layer. Was it sold 'as seen' (how does that work over the internet)? It strikes me, you have some case for compensation.

Regards

Chas