Apply Coupon Code To Subtotal NOT GRAND TOTAL

Coupons in hikashop are always used on the taxed price.
The discount before/after tax option is only for discounts.
One solution would be to create a discount and restrict it to a group of joomla users using the access level feature of hikashop business. Then, you could move that user to that group of users so that the discount is automatically applied to your products prices.

Another option could be this:
Edit your tax rule so that it apply only to individuals. Then, in the config make sure that the default user type is individual.
Finally, in Display->Custom file, publish the file address_company, remove it from the front end, and change its type to be a radio button.
That way, you could ask your users to register to our website via the hikashop registration screen so that they enter their company address (or you could do it for them), and on your end, you would be able to activate their address as being a company address. That way, the tax will automatically calculated to remove the VAT on the checkout, once they select their address.