Clearing and loading values on HTML form fields
Good question,

you can clear all (visible) input elements like this:

$("input:text").is(":visible").val('');


In general, google queries are a gold mine here. Just google 'jQuery clear all text fields' and you will find a link to StackOverflow with several approaches.

You are correct that there are easier ways to get your response in the correct fields, something like this might work for you:

}).done(function(response) {
    Object.keys(response).forEach( function( field ){
      $('#'+field).val(response[field]);
    });
    var updateInfo = "";        
    ...
    $('#updateInfo').html(updateInfo);
    $('#modal').modal('show');
});


As for #3 you should look into libraries to that work for you. A number of folks have done all the heavy lifting for that already.