I have to run a PHP script through real cron (WP cron being too unreliable). Within that script, I need $wpdb to insert data into WordPress table. But of course $wpdb will not be available as WordPress would not be initialized. Right? So, my question is how to 'include' WordPress/initialize WordPress environment to do such tasks? How about require_once("wp-load.php")?
You can use real cron to trigger WP cron - by fetching
wp-cron.php file from root (snippet from quick google search).
That will take care of environment and everything.
If you want to trigger any script via cron you can also skip wp-cron (limited functionality anyway) and cron the script directly.
The script have to load the wordpress environment itself if it wants to use wordpress functionality.
A solution to this can be found here : Initialize WordPress environment to use in command line script