Hi,
There is no back button option, you can potentially add one button by editing the view "checkout / step" and add a button with some javascript for a redirect back. But this can cause issue because from one page to another a form is submitted so it can potentially display an error page.
Note that if you display the checkout progress bar, the customers are able to go back by just clicking on the desired step.
For the other issue, you just have to disable the option "Auto submit shipping and payment methods selection" in Configuration > Checkout.