Taxes should be set as default tax amount, and the other options should be set as exemptions. This would be the easiest way to resolve the issue and loophole. When setting up taxes, in the commerce section, there should be an option to set up 'default' taxes collected. Then the Add Country option would be overrides and exemptions where the tax amount is set based on the shipping address.
I believe this is the simplest way to resolve this. I don't think it would be a major recoding on your side of things, and it would lock down this loophole.