Unfortunately both the missing law tag and the stain would invalidate the warranty.
I think your odds of making a successful warranty claim are next to nothing and I don’t think that hiring a lawyer would make much difference since since their warranty conditions are clearly listed and not having a law tag and a stain on your mattress would both invalidate the warranty on almost every mattress in the industry.
Post #4 here may be worth reading though and as a last resort it may be worth considering mattress surgery.