How to switch between conflictual hooks?
If you implement a hook, it will always be executed.

The condition of a query string makes things more complicated in your situation, because all menu items are defined by their path (taxonomy/term/tid in your case). This means that - as far as Drupal is concerned - taxonomy/term/tid?v=value and taxonomy/term/tid have the same menu item, so both requests will be handled by the same code.

Is it really necessary that the URL takes the form of ?v=value? If you could make it taxonomy/term/tid/value, you can create your own custom menu item for taxonomy/term/%/%, and simply leave the taxonomy/term/tid path as is.