1) You can add that in DW after the page is generated
2) Just change the label after it is created, that shouldn't be difficult... don't bother to change the form element names, that would add unnecessary complication.
3) Instead of removing it... make the input type="text" into type="hidden" and set its value to the country you want to use. You can move the hidden form element to a different row and delete the row so it will appear as if it is completely removed.
Captcha requires the GD library enabled on your php server... that is most likely the problem when viewing locally. You should be able to enable GD2 in your php.ini very easily... read more about the GD library at www.php.net.