Hi,
1. That's because the file "show_block_terms" has been customized with an old version of HikaShop 3 and we had to do crutial modifications to it to support multiple terms and conditions views in the checkout since then.
If you rename the file templates/YOUR_TEMPLATE/html/com_hikashop/checkout/show_block_terms.php that will deactivate the view override and it will work.
So you can then reapply the customization you did to it to the new version of the view file.
If it's not you who did the customization and it comes from your template, you'll have to check with your template provider. Maybe just updating your template would be enough.
2. I didn't get the problem. However, looking at the HTML of the area I can see that the acymailing checkbox is not being added at the correct place in the registration form. It's added inside the VAT line area instead of after it:
monosnap.com/file/ObgUzWQeZlSQSJE3UuRA4LCSTeE55R
So it might be why you have some problem with it.
This might be linked to the customization made on the registration form or maybe a bug in the way the acymailing (auto)subscribe plugin adds the checkbox to the registration form.
You should first check on that issue with the support of AcyMailing. Also, make sure you tell them precisely how to reproduce the issue.