I did read your entire question, I changed the subject to better reflect the question being asked.
This is not a bug in eCart5, rather it is a problem with taxes being applied.
The taxes are based on the Billing state that is entered in the checkout form on the checkout page.
When the checkout form posts to the confirm page, the billing state is stored in a session variable.
The tax calculation is triggered on 1) The existence of this session variable and 2) The value of the session variable.
If it is set to the correct state, the tax will display any time you view the cart page.
It cannot display the tax before going through the checkout page to the confirm page, because the billing state is not known yet.
This is not a bug in eCart, it is the way it is supposed to work.