Hi
You can enter the tax exemption numbers for the client under People > Customers. Then if you use a custom tax rule, you can enable the Use tax exemption number to exempt a customer from taxes.
1. During checkout, the customer billing address is available to you in the custom rule to query so you can create a rule to charge/exempt based on the information. In the next release, we'll include the IP address so you can query in a custom rule. In addition, that custom rule can use Geo IP location to detect the country based on the IP.
2. Yes, if the customer has a tax exemption number, you can create a rule to exempt the taxes in the XSLT.
I hope this helps. Thanks,