There was a lot of customization in place that was causing the cart to not work, at the end of the day, you had removed the client validation, but not the server validation from the checkout form and that was causing issues.
there is still an issue with the credentials, I think it may be the & in your password, you should try changing the password to something that doesn't include the & character. if you need further help In getting this to work, I would suggest signing up for a premiere support appointment given the amount of customization you have done in the code.