1,2 – best way to change strings while using English would be to change in wp-config language to “en_GB” than modify “en_GB” language under codestyling localisation plugin.
3 – there is no easy way for that for now. If you’d like to have quote for custom development of such feature – please contact us via support@jigoshop.com
For 1 and 2 – but i changed the words for other terms using plugin file editing. I use (by the way) Russian language. I’ve renamed Out of stock and In stock terms on the product page (….product.php), but i don’t understand what php file is responsible for Out of stock phrase on main shop page, where the file responsible for cart widget (button names).
3. How much will it cost to develop an update?
1,2 – you really shouldn’t change core files – you’d loose your changes after upgrade. Simply use Codestyling Localisation plugin in that case.
3 – please provide your site url for check via support@jigoshop.com