Be careful of using dish soap. Some of them contain bleach and you may not want that.
I once had a pale peach cotton outfit. I got a coffee stain on it. Someone told me to use dish soap. So I put some on and while I was letting it soak, I told someone on the phone what I was doing. She asked me if I wan't afraid I'd bleach the fabric. Sure enough, now I had white spots on it.
Since it was now white, I decided to bleach the whole outfit and have a white outfit instead of peach. Well, it was sewn with peach poly thread...the fabric all bleached out white but all the threads were showing. So I decided to color it peach with Rit dye.
I put it in the pot and dyed it and it came out an ugly orange. So I put it back in the washing machine with bleach and it came out just perfect.
So just keep trying stuff until you are happy. But, I don't think your quilt is ruined. This quilt is beautiful and I'd use it as is. The spot would make a good conversation piece.