You have a great customised desktop new item aspx form but you also want to have a Nintex mobile form available.
Publishing a Nintex Mobile form will overwrite your Desktop form as the Desktop form is the default form and cannot be removed.

To maintain the current desktop form but also have a Nintex mobile form you can create a second content type on the list and then create a Nintex Form on the second content type.

Desk top users will still have there form and Mobile users will only see the Mobile form.