Your JavaScript error is a Braintree bug: «Uncaught TypeError: Cannot read property ‘addErrorMessage’ of undefined».
I think it is not related to my Stripe extension, but I have fixed it.
Maybe you have not encounter it before just because you do not re-deploy the static content so often.
I have fixed the «Can only create one Element of type cardNumber» issue.
This issue was caused by the HS_OneStepCheckout module.
It reloads the payment form on country changing.
Unfortunately, there is no way to preserve the previously entered bank card data after such reload, because Stripe dynamically creates an IFRAME for each bank card form element, and it does not allow any JavaScript to access the entered values (for security reasons).