Is it possible to set up a path to upload folder outside WP (eg. root folder)? no, currently this is not an option, all the galleries ge uploaded to the wp_content/{YOUR_CUSTOM_FOLDER}

is it necessary to secure the folder using .htaccess? is not necessary but you can remove GETand POST requests to the folder. the plugin uses a key value to encode/hide the real image path

exactly mean in the installation manual by changing upload folder in settings?
you can change the name of the folder where the images get uploaded {YOUR_CUSTOM_FOLDER}, go to settings>galleries>Gallery folder path. you can also change the path in each gallery in the information box.

are you considering in the future to include in this plugin support for Amazon S3 storage services? this could be a great option, currently I don’t have an account to test it but I will added to my future feature.