I recommend you sending it out to a professional. The adding of different chemicals can have a reverse reaction to this satin bag. Often we think we can do it ourselves. I would not take the chance anymore , you have tried. Now give the pros a chance. Try going with a professional that may have a guarantee. Also you can contact Chanel , sometimes they will help with the care of your purse.
You undoubtedly have a lovely purse and having it cleaned by a professional would be the best choice but, please, do not entrust this clean up to anyone but someone that Chanel recommends as I have heard horror about 'local' professionals.
I would suggest you take it to the store where purchased if possible and ask them for help as they many times can send it to Chanel or recommend someone for you to take it to.
If no way to go to original store then go to any store that sells Chanel (not an outlet) and ask for help.
They can give you a direct Chanel address/number for you to call.
I would stop doing it yourself and take it to a professional. I think s/he could get the cornstarch out, at the very least. You do not want to devalue the bag by doing something wrong.
If it is a real Chanel bag, I would ask them:
I don't know if they still, but they used to have an internal repair service.
Post back what you learn!