Indeed, if you set a zone restriction for a plugin and that your default zone is outside that zone, the plugins won't be displayed by default. Then, when the user selects his address, the plugins zone restriction will be checked with his address instead of the default zone. It's the normal behavior.
When the user changes the quantity, the page is automatically refreshed to display the cart again with the new price. Instead of having to click on another button, it seems easier for the user which will have one less click to make. You can always remove that behavior if you don't like it. You can edit the file cart of the view checkout via the menu Display->Views. There you will have the code
onchange="this.form.submit(); return false;"
which handles the quantity update. You can just remove it.