Hi,
Install the new version, and turn off the "legacy checkout" setting in the HikaShop configuration to switch to the new checkout.
If you still have a blank page error, turn on the "error reporting" and the "debug mode" settings of the Joomla configuration and try again. You should then get an error message on that blank page which will help understanding what is going on.
Regarding the T&C checkbox, if it doesn't work, please check if you have the file templates/YOUR_TEMPLATE/html/com_hikashop/checkout/show_block_terms.php and if you do, rename it in order to deactivate it.
Regarding the coupon submit button, I can think or either a translation override issue or a CSS problem. Hard to say anything more without being able to look at the issues directly on your website.