Hi,
1. In order to add a date selector field on your checkout, you can create a new field of the table "order" and of the type "date" via the menu Display->Custom fields (Business edition only). It should appear automatically on the checkout if you didn't remove the 'fields' view from your checkout workflow.
2. I would recommend to enable the HikaShop cart module via the Joomla modules manager. Also, in the HikaShop options screen of that module, you'll find an option to activate the "mini cart" mode and in that case you'll have an icon to clear all the cart if you turn on the "show delete button" option there.
3. Create a tax rule via the menu System>Taxes and do not specify any "zone" for your tax rule and it will apply to everyone, regardless of their address.